@comment -*-texinfo-*-
@comment this file contains the general information about Singular

@c The following directives are necessary for proper compilation
@c with emacs (C-c C-e C-r).  Please keep it as it is.  Since it
@c is wrapped in `@ignore' and `@end ignore' it does not harm `tex' or
@c `makeinfo' but is a great help in editing this file (emacs
@c ignores the `@ignore').
@ignore
%**start
\input texinfo.tex
@setfilename general.info
@node Top, General concepts
@menu
* General concepts::
@end menu
@node General concepts, Data types, Introduction, Top
@chapter General concepts
%**end
@end ignore

@menu
* Interactive use::
* Emacs user interface::
* Rings and orderings::
* Implemented algorithms::
* The SINGULAR language::
* Input and output::
* Procedures::
* Libraries::
* Debugging tools::
* Dynamic loading::
@end menu


@c ---------------------------------------------------------------------------
@node Interactive use, Emacs user interface, General concepts, General concepts
@section Interactive use
@cindex Interactive use

In this section, aspects of interactive use are discussed.  This
includes how to enter and exit @sc{Singular}, how to interpret its
prompt, how to get online help, and so on.

There are a few important notes which one should not forget:
@itemize @bullet
@item
every command has to be terminated by a @code{;} (semicolon) followed
by a @key{RETURN}
@item
the online help is accessible by means of the @code{help} function
@end itemize

@menu
* How to enter and exit::
* The SINGULAR prompt::
* The online help system::
* Interrupting SINGULAR::
* Editing input::
* Command line options::
* Startup sequence::
@end menu

@c --------------------------------------------------------------------------
@node How to enter and exit, The SINGULAR prompt, Interactive use, Interactive use
@subsection How to enter and exit
@cindex How to enter and exit

@sc{Singular} can either be run in an ASCII-terminal or within Emacs.

To start @sc{Singular} in its ASCII-terminal user interface,  enter
@code{Singular} at the system prompt. The @sc{Singular} banner appears which,
among other data, reports the version and the compilation date.

To start @sc{Singular} in its Emacs user interface, either enter
@code{ESingular} at the system prompt, or type @code{M-x singular}
within a running Emacs (provided you have loaded the file
@code{singular.el} in your running Emacs, see @ref{Running SINGULAR
under Emacs} for details).

Generally, we recommend to use @sc{Singular} in its Emacs interface,
since this offers many more features and is more convenient to
use than the ASCII-terminal interface (@pxref{Emacs user interface}).

To exit @sc{Singular} type @code{quit;}, @code{exit;} or @code{$} (or,
when running within Emacs preferably type @code{C-c $}).

@sc{Singular} and @code{ESingular} may also be started with command line
options and with
filenames as arguments. More generally, the startup syntax is
@smallexample
Singular  [options] [file1 [file2 @dots{}]]
ESingular  [options] [file1 [file2 @dots{}]]
@end smallexample

@xref{Command line options}, @ref{Startup sequence}, @ref{Running SINGULAR
under Emacs}.


@c --------------------------------------------------------------------------
@node The SINGULAR prompt,The online help system,How to enter and exit, Interactive use
@subsection The SINGULAR prompt
@cindex prompt
@cindex error recovery
The @sc{Singular} prompt @code{>} (larger than) asks the user for input
of commands.  The ``continuation'' prompt @code{.} (period) asks the
user for input of missing parts of a command (e.g. the semicolon at
the end of every command).

@sc{Singular} does not interpret the semicolon as the end of a command
if it occurs inside a string.  Also, @sc{Singular} waits for blocks
(sequences of commands enclosed in curly brackets) to be closed before
prompting with @code{>} for more commands.  Thus, if @sc{Singular} does
not respond with its regular prompt after typing a semicolon it may wait
for a @code{"} or a @code{@}} first.

Additional semicolons will not harm @sc{Singular} since they are
interpreted as empty statements.

@c --------------------------------------------------------------------------
@node The online help system,Interrupting SINGULAR, The SINGULAR prompt,Interactive use
@subsection The online help system
@cindex The online help system
@cindex online help
@cindex help, online help system

The online help system is invoked by the @code{help} command.
@code{?} may be used as a synonym for @code{help}.  Simply typing
@code{help;} displays the ``top'' of the help system (i.e., the title
page of the @sc{Singular} manual) which offers a
short table of contents.  Typing @code{help} topic@code{;} shows the
available documentation on the respective topic.  Here, topic may be either a function
name or, more generally, any index entry of the @sc{Singular}
manual. Furthermore, topic may contain wildcard characters.
@xref{help}, for more information.

@cindex browsers
@cindex help browsers
Online help information can be displayed in various help browsers. The
following table lists a summary of the browsers which are always present.
Usually, external browsers are much more convenient:
A complete, customizable list can be found in the file @code{LIB/help.cnf}.

@multitable @columnfractions .2 .2 .6
@item @strong{Browser} @tab @strong{Platform}
@tab @strong{Description}

@item html
@cindex help browsers, html
@cindex html, default help
@tab Windows
@tab displays a html version of the manual in your default html browser

@item builtin
@cindex help browsers, dummy
@tab all
@tab simply outputs the help information in plain ASCII format

@item emacs
@cindex help browsers, emacs
@tab Unix, Windows
@tab when running @sc{Singular} within (X)emacs, displays help inside the
(X)emacs info buffer.

@item dummy
@tab all
@cindex help browsers, dummy
@tab displays an error message due to the non-availability of a help browser
@end multitable

External browsers depend on your system and the contents of @code{LIB/help.cnf},
the default includes:
@code{htmlview} (displays HTML help pages via @code{htlmview}),@*
@code{mac} (displays HTML help pages via @code{open}),@*
@code{mac-net} (displays HTML help pages via @code{open}),@*
@code{mozilla} (displays HTML help pages via @code{mozilla}),@*
@code{firefox} (displays HTML help pages via @code{firefox}),@*
@code{konqueror} (displays HTML help pages via @code{konqueror}),@*
@code{galeon} (displays HTML help pages via @code{galeon}),@*
@code{netscape} (displays HTML help pages via @code{netscape}),@*
@code{safari} (displays HTML help pages on MacOsX via @code{safari}),@*
@code{tkinfo} (displays INFO help pages via @code{tkinfo}),@*
@code{xinfo} (displays INFO help pages via @code{info}),@*
@code{info} (displays INFO help pages via @code{info}),@*
@code{lynx} (displays HTML help pages via @code{lynx}).

The browser which is used to display the help information, can be either
set at startup time with the command line option  (@pxref{Command line
options})
@smallexample
--browser=<browser>
@end smallexample
or with the @sc{Singular} command (@pxref{system})
@smallexample
system("--browser", "<browser>");
@end smallexample
The  @sc{Singular} command
@smallexample
system("browsers");
@end smallexample
lists all available browsers and the command
@smallexample
system("--browser");
@end smallexample
returns the currently used browser.

If no browser is explicitly set by the user, then the first available browser
(w.r.t. the order of the browsers in the file @code{LIB/help.cnf}) is chosen.

The @code{.singularrc} (@pxref{Startup sequence}) file is a good place
to set your default browser. Recall that if a file
@code{$HOME/.singularrc} exists on your system, then the content of this
file is executed before the first user input. Hence, putting
@smallexample
if (! system("--emacs"))
@{
  // only set help browser if not running within emacs
  system("--browser", "info");
@}
// if help browser is later on set to a web browser,
// allow it to fetch HTML pages from the net
system("--allow-net", 1);
@end smallexample
in your file @code{$HOME/.singularrc} sets your default browser to
@code{info}, unless @sc{Singular} is run within emacs (in which case the
default browser is automatically set to @code{emacs}).

Obviously, certain external files and programs are required for the
@sc{Singular} help system to work correctly. If something is not available
or goes wrong, here are some tips for troubleshooting the help system:

@itemize @bullet
@item
@cindex DISPLAY environment variable
@cindex environment variable, DISPLAY
Under Unix, the environment variable @code{DISPLAY} has to be set for all X11
browsers to work.

@item
@cindex help browsers, setting command to use
The help browsers are only available if the respective programs are installed
on your system (for @code{xinfo}, the programs @code{xterm} and
@code{info} are necessary). You can explicitly specify which program to
use, by changing the entry in @code{LIB/help.cnf}

@item
If the help browser cannot find the local html
pages of the @sc{Singular} manual (which it will look for at
@code{$RootDir/html} -- see @ref{Loading a library} for more info on
@code{$RootDir}) @emph{and} the (command-line) option @code{--allow-net}
has @emph{explicitly} been set (see @ref{Command line options} and
@ref{system} for more info on
setting values of command-line options), then it dispatches the html
pages from
@url{https://www.singular.uni-kl.de/Manual}. (Note that
the non-local net-access of HTML pages is disabled, by default.)
@*An
alternative location of a local directory where the html pages reside
can be specified by setting the environment variable
@code{SINGULAR_HTML_DIR}.
@c TBC (@xref{Setting environment variables}).

@item
The @code{info} based help browsers @code{tkinfo}, @code{xinfo}, @code{info}, and
@code{builtin} need the (info) file @code{singular.hlp} which will be looked
for at @code{$RootDir/info/singular.hlp} (see @ref{Loading a library}
for more info on @code{$RootDir}).  An alternative
location of the info file of the manual can be specified by setting the
environment variable @code{SINGULAR_INFO_FILE}.
@c TBC (@xref{Setting environment variables}).
@end itemize

@c ref
@ref{Command line options}
@ref{Loading a library}
@ref{system}
@ref{Startup sequence}
@c ref

@subsubheading Info help browsers
@cindex info

The help browsers @code{tkinfo}, @code{xinfo} and @code{info} (so-called
info help browsers) are based on the
@code{info} program from the GNU @code{texinfo} package.  @xref{Top, An
Introduction to Info, Getting started, info, The Info Manual}, for more
information.

For info help browsers, the online manual is decomposed into ``nodes''
of information, closely
related to the division of the printed manual into sections and
subsections.  A node contains text describing a specific topic at a
specific level of detail.  The top line of a node is its ``header''.
The node's header tells the name of the current node (@code{Node:}), the
name of the next node (@code{Next:}), the name of the previous node
(@code{Prev:}), and the name of the upper node (@code{Up:}).

To move within info, type commands consisting of single characters.  Do
not type @code{RETURN}.  Do not use cursor keys, either.  Using some of
the cursor keys by accident might pop to some totally different node.
Type @code{l} to return to the original node.  Some of the @code{info}
commands read input from the command line at the bottom.  The
@code{TAB} key may be used to complete partially entered input.

The most important commands are:
@table @asis
@item @code{q}
leaves the online help system
@item @code{n}
goes to the next node
@item @code{p}
goes to the previous node
@item @code{u}
goes to the upper node
@item @code{m}
picks a menu item specified by name
@item @code{f}
follows a cross reference
@item @code{l}
goes to the previously visited node
@item @code{b}
goes to the beginning of the current node
@item @code{e}
goes to the end of the current node
@item @code{SPACE}
scrolls forward a page
@item @code{DEL}
scrolls backward a page
@item @code{h}
invokes info tutorial (use @code{l} to return to the manual or
@code{CTRL-X 0} to remove extra window)
@item @code{CTRL-H}
shows a short overview over the online help system (use @code{l} to return
to the manual or @code{CTRL-X 0} to remove extra window)
@item @code{s}
searches through the manual for a specific string, and selects the node in
which the next occurrence is found
@item @code{1}, @dots{}, @code{9}
picks i-th subtopic from a menu
@end table

@c --------------------------------------------------------------------------
@node Interrupting SINGULAR,Editing input,The online help system,Interactive use
@subsection Interrupting SINGULAR
@cindex Interrupting SINGULAR

On Unix-like operating systems and on Windows NT, typing @code{CTRL-C}
(or, alternatively @code{C-c C-c}, when running within Emacs),
interrupts @sc{Singular}.  @sc{Singular} prints the current command and
the current line and prompts for further action.  The following choices
are available:
@table @code
@item a
returns to the top level after finishing the current (kernel) command. Notice
that commands of the @sc{Singular} kernel (like @code{std}) cannot be
aborted, i.e. (@code{a})bort only happens whenever the interpreter is active.
@item c
continues
@item q
quits  @sc{Singular}
@end table

@c --------------------------------------------------------------------------
@node Editing input,Command line options,Interrupting SINGULAR,Interactive use
@subsection Editing input
@cindex Editing input
@cindex SINGULARHIST

@c This section describes only a subset of the key bindings of
@c @sc{Singular} binaries built with the GNU Readline library.
@c @xref{Command Line Editing, GNU Readline Library, Command Line Editing,
@c readline, The GNU Readline Library Manual}, for more information.

The following keys can be used for editing the input and retrieving
previous input lines:
@table @code
@item TAB
provides command line completion for function names and file names
@item CTRL-B
moves cursor to the left
@item CTRL-F
moves cursor to the right
@item CTRL-A
moves cursor to the beginning of the line
@item CTRL-E
moves cursor to the end of the line
@item CTRL-D
deletes the character under the cursor
@* Warning: on an empty line, @code{CTRL-D} is interpreted as the
@code{EOF} character which immediately terminates @sc{Singular}.
@item BACKSPACE
@itemx DELETE
@itemx CTRL-H
deletes the character before the cursor
@item CTRL-K
kills from cursor to the end of the line
@item CTRL-U
kills from cursor to the beginning of the line
@item CTRL-N
saves the current line to history and gives the next line
@item CTRL-P
saves the current line to history and gives the previous line
@item RETURN
saves the current line to the history and
sends it to the @sc{Singular} parser for interpretation
@end table

When run under a Unix-like operating system and in its ASCII-terminal
user interface,   @sc{Singular} tries to dynamically link at runtime
with the GNU Readline library. @xref{Command Line Editing, GNU Readline
Library, Command Line Editing, readline, The GNU Readline Library
Manual}, for more information. If a shared version of this library can
be found on your machine, then additional command-line editing
features like  history completion are available.
In particuliar, if @sc{Singular} is able to load that library and
if the environment
variable @code{SINGULARHIST} is set and has a name of a valid file as value,
then the input history is stored across sessions using this
file. Otherwise, i.e., if the environment variable @code{SINGULARHIST}
is not set, then the history of the last inputs is only available for
previous commands of the current session.

@c --------------------------------------------------------------------------
@node Command line options, Startup sequence, Editing input, Interactive use
@subsection Command line options
@cindex Command line options

The startup syntax is
@smallexample
Singular  [options] [file1 [file2 @dots{}]]
ESingular  [options] [file1 [file2 @dots{}]]
@end smallexample
Options can be given in both their long and short format. The following
options control the general behaviour of @sc{Singular}:
@table @asis

@item @code{-d}, @code{--sdb}
@cindex -d
@cindex --sdb
@cindex Source code debugger, invocation
Enable the use of the source code debugger.
@xref{Source code debugger}.

@item @code{-e}, @code{--echo[=VAL]}
@cindex -e
@cindex --echo
Set value of variable @code{echo} to @code{VAL} (integer in the range
0, @dots{}, 9). Without an argument, @code{echo} is set to 1, which echoes
all input coming from a file. By default, the value of @code{echo} is
0. @xref{echo}.

@item @code{-h}, @code{--help}
@cindex -h
@cindex --help
@cindex command-line options, short help
Print a one-line description of each command line option and exit.

@item @code{--allow-net}
@cindex --allow-net
@cindex net access
@cindex allowing net access
@cindex help, accessing over the net
Allow the help browsers based on a web browser to fetch HTML manual pages over
the net from the WWW home-site of @sc{Singular}. @xref{The online help
system}, for more info.

@item @code{--browser="VAL"}
@cindex --browser
@cindex browser, command line option
Use @code{VAL} as browser for the @sc{Singular} online manual.
@*@code{VAL} may be one of the browsers mentioned in @code{LIB/help.cnf},
for example @code{html} (Windows only), @code{mozilla}, @code{firefox},
@code{konqueror}, @code{galeon}, @code{netscape}, @code{safari} (OsX only),
@code{xinfo}, @code{tkinfo}, @code{info}, @code{builtin}, or @code{emacs}.
Depending on your platform and local
installation, only some browsers might be available. The default browser
is @code{html} for Windows and one based on a web browser for Unix
platforms. @xref{The online help system}, for more info.

@item @code{--no-rc}
@cindex --no-rc
@cindex .singularrc file, no loading
Do not execute the @code{.singularrc} file on start-up.  By default,
this file is executed on start-up.  @xref{Startup sequence}.

@item @code{--no-stdlib}
@cindex --no-stdlib
Do not load the library @code{standard.lib} on start-up. By default,
this library is loaded on start-up.  @xref{Startup sequence}.

@item @code{--no-warn}
@cindex --no-warn
Do not display warning messages.

@item @code{--no-out}
@cindex --no-out
Suppress display of all output.

@item @code{--no-shell}
@cindex --no-shell
Runs Singular in restricted mode to disallow shell escape commands.
Objects of type link will also be unable to use.

@item @code{-t}, @code{--no-tty}
@cindex --no-tty
Do not redefine the characteristics of the terminal.  This option should be
used for batch processes.

@item @code{-q}, @code{--quiet}
@cindex -q
@cindex --quiet
Do not print the start-up banner and messages when loading
libraries. Furthermore, redirect @code{stderr}
(all error messages) to @code{stdout} (normal output channel).  This
option should be used if @sc{Singular}'s output is redirected to a file.

@item @code{-v}
@cindex -v
Print extended information about the version and configuration of
@sc{Singular}  (used optional parts, compilation date, start of random
generator etc.). This information should be included if a user reports
an error to the authors.
@*It also list all the used directories/files
(see @ref{Used environment variables}).
@end table

The following command line options allow manipulations of the timer and
the pseudo random generator and enable the passing of commands and strings
to @sc{Singular}:

@table @asis
@item @code{-c}, @code{--execute=STRING}
@cindex -c
@cindex --execute
Execute @code{STRING} as (a sequence of) @sc{Singular} commands on
start-up after the @code{.singularrc} file is executed, but prior to
executing the files given on the command line.  E.g., @code{Singular -c
"help all.lib; quit;"} shows the help for the library @code{all.lib} and
exits.

@item @code{-u}, @code{--user-option=STRING}
@cindex -u
@cindex --user-option
Returns @code{STRING} on @code{system("--user-option")}. This is useful
for passing arbitrary arguments from the command line to the
@sc{Singular} interpreter.  E.g.,
@*@code{Singular -u "xxx.dump" -c 'getdump(system("--user-option"))'}
reads the file @code{xxx.dump} at
start-up and allows the user to start working with all the objects
defined in a previous session.

@item @code{-r}, @code{--random=SEED}
@cindex -r
@cindex --random
Seed (i.e., set the initial value of) the pseudo random generator with
integer @code{SEED}.  If this option is not given, then the random
generator is seeded with a time-based @code{SEED} (the number of
seconds since January, 1, 1970, on Unix-like operating systems, to be
precise).

@item @code{--min-time=SECS}
@cindex --min-time
If the @code{timer} (@pxref{timer}), resp.@:  @code{rtimer}
(@pxref{rtimer}) , variable is
set, report only
times larger than @code{SECS} seconds (@code{SECS} needs to be a
floating point number greater than 0).  By default, this value is set to
0.5 (i.e., half a second).  E.g., the option @code{--min-time=0.01}
forces @sc{Singular} to report all times larger than 1/100 of a
second.

@item @code{--ticks-per-sec=TICKS}
@cindex --ticks-per-sec
Set unit of timer to @code{TICKS} ticks per second (i.e., the value
reported by the @code{timer} and @code{rtimer} variable divided by
@code{TICKS} gives the time in seconds).  By default, this value is 1.

@item @code{--cpus=CPUs}
set the maximal number of CPUs to use.

@item @code{--cntrlc=C}
set the default answer for interrupt signals to C which should be
a for abort, c for continue or q for quit.
@end table

The next three options are of interest for the use with ssi links:
@table @asis
@item @code{-b}, @code{--batch}
@cindex -b
@cindex --batch
Run in batch mode. Opens a TCP/IP connection with host specified by
@code{--MPhost} at the port specified by @code{--MPport}. Input is read
from and output is written to this connection in the format given by @code{--link}.
@xref{Ssi links}.

@item @code{--MPport=PORT}
@cindex --MPport
Use @code{PORT} as default port number for connections (whenever not
further specified). This option is mandatory when the @code{--batch}
option is given.  @xref{Ssi links}.

@item @code{--MPhost=HOST}
@cindex --MPhost
Use @code{HOST} as default host for connections (whenever not
further specified).  This option is mandatory when the @code{--batch}
option is given.  @xref{Ssi links}.
@end table

Finally, the following options are only available when running
@code{ESingular} (see @ref{Running SINGULAR under Emacs} for details).

@table @code

@item --emacs=EMACS
@cindex --emacs
Use @code{EMACS} as Emacs program to run the @sc{Singular} Emacs
interface, where @code{EMACS} may e.g. be emacs or xemacs.

@item --emacs-dir=DIR
@cindex --emacs-dir
Set the singular-emacs-home-directory, which is the directory where
singular.el can be found, to @code{DIR}.

@item --emacs-load=FILE
@cindex --emacs-load
Load @code{FILE} on Emacs start-up, instead of the default load file.

@item --singular=PROG
@cindex --singular
Start @code{PROG} as @sc{Singular} program within Emacs
@end table

The value of options given to @sc{Singular} (resp.@: their default values,
if an option was not given), can be checked with the command
@code{system("--}long_option_name@code{")}.  @xref{system}.
@smallexample
@c example
  system("--quiet");    // if ``quiet'' 1, otherwise 0
  system("--min-time"); // minimal reported time
  system("--random");   // seed of the random generator
@c example
@end smallexample

Furthermore, the value of options (e.g., @code{--browser}) can be
re-defined while @sc{Singular} is running using the command
@code{system("--}long_option_name_string @code{",}expression@code{)}. @xref{system}.
@smallexample
  system("--browser", "builtin");  // sets browser to 'builtin'
  system("--ticks-per-sec", 100);  // sets timer resolution to 100
@end smallexample


@c --------------------------------------------------------------------------

@node Startup sequence, , Command line options, Interactive use
@subsection Startup sequence
@cindex Startup sequence

On start-up, @sc{Singular}
@enumerate
@item
loads the library @code{standard.lib} (provided the @code{--no-stdlib}
option was not given),

@item
@cindex file, .singularrc
@cindex .singularrc file
@cindex singularrc
searches the current directory and then the
home directory of the user, and then all directories contained in the
library @code{SearchPath} (see @ref{Loading a library} for more info
on @code{SearchPath}) for a file named @code{.singularrc} and
executes it, if found (provided the @code{--no-rc} option was not
given),

@item
executes the string specified with the @code{--execute} command line
option,

@item
executes the files @code{file1}, @code{file2} @dots{}  (given on the command
line) in that order.
@end enumerate

@strong{Note:} @code{.singularrc} file(s) are an appropriate place for
setting some default values of (command-line) options.

For example, a system administrator might remove the locally installed
HTML version of the manual and put a @code{.singularrc} file
with the following content
@smallexample
if (system("version") >= 1306) // assure backwards-compatibility
@{
  system("--allow-net", 1);
@}; // the last semicolon is important: otherwise no ">", but "." prompt
@end smallexample
in the directory containing the @sc{Singular} libraries, thereby
allowing to fetch the HTML on-line help from the WWW home-site of
@sc{Singular}.

On the other hand, a single user might put a @code{.singularrc} with the
following content
@smallexample
if (system("version") >= 1306) // assure backwards-compatibility
@{
  if (! system("--emacs"))
  @{
    // set default browser to info, unless we run within emacs
    system("--browser", "info");
  @}
@}; // the last semicolon is important: otherwise no ">", but "." prompt
@end smallexample
in his home directory, which sets the default help browser to
@code{info} (unless @sc{Singular} is run within emacs) and thereby
prevents the execution of the"global"
@code{.singularrc} file installed by the system administrator (since the
@code{.singularrc} file of the user is found before the "global"
@code{.singularrc} file installed by the system administrator).

@c ---------------------------------------------------------------------------
@node Emacs user interface, Rings and orderings, Interactive use, General concepts
@section Emacs user interface
@cindex Emacs, user interface
@cindex Emacs
@cindex interface, Emacs
@cindex user interface, Emacs

Besides running @sc{Singular} in an ASCII-terminal, @sc{Singular} might
also be run within Emacs. Emacs (or, XEmacs which is very similar) is a
powerful and freely available text editor, which, among others,
provides a framework for the implementation of interactive user
interfaces. Starting from version 1.3.6, @sc{Singular} provides such an
implementation, the so-called @sc{Singular} Emacs mode, or Emacs user
interface.

Generally, we recommend to use the Emacs interface,
instead of the ASCII-terminal interface: The Emacs interface does not
only provide everything the ASCII-terminal interface provides, but
offers much more. Among others, it offers
@itemize @bullet
@item
color highlighting
@item
truncation of long lines
@item
folding of input and output
@item
TAB-completion for help topics
@item
highlighting of matching parentheses
@item
key-bindings and interactive menus for most user interface commands and
for basic @sc{Singular} commands (such as loading of libraries and files)
@item
a mode for running interactive @sc{Singular} demonstrations
@item
convenient ways to edit @sc{Singular} input files
@item
interactive customization of nearly all aspects of the user-interface.
@end itemize

In order to use the @sc{Singular}-Emacs interface you need to have Emacs version
20 or higher, or XEmacs
version 20.3 or higher installed on your system. These
editors can be downloaded for most hard- and software platforms,
sources from either
@uref{http://www.gnu.org/software/emacs/emacs.html} (Emacs), or from
@uref{http://www.xemacs.org} (XEmacs).
(Download of binaries depend on your OS). The
differences between Emacs and XEmacs w.r.t. the  @sc{Singular}-Emacs
interface are marginal -- which editor to use is mainly a matter of personal
preferences.

The simplest way to start-up @sc{Singular} in its Emacs interface is by
running the program @code{ESingular} which is contained in the Singular
distribution. Alternatively, @sc{Singular} can be started within an
already running Emacs -- see @ref{Running SINGULAR under Emacs} for
details.

The next section gives  a tutorial-like introduction to Emacs. This
introductory section is
followed by sections which explain the functionality of various aspects
of the Emacs user interface in more detail: how to start/restart/kill
@sc{Singular} within Emacs, how to run an interactive demonstration, how
to customize the Emacs user interface, etc. Finally, the 20 most
important commands of the Emacs interface together with their key
bindings are listed.

@menu
* A quick guide to Emacs::
* Running SINGULAR under Emacs::
* Demo mode::
* Customization of the Emacs interface::
* Editing SINGULAR input files with Emacs::
* Top 20 Emacs commands::
@end menu

@c --------------------------------------------------------------------------
@node A quick guide to Emacs, Running SINGULAR under Emacs, Emacs user interface, Emacs user interface
@subsection A quick guide to Emacs
@cindex Emacs, overview
@cindex Emacs, a quick guide

This section gives a tutorial-like introduction to Emacs. Especially to
users who are not familiar with Emacs, we recommend that they go through
this section and try out the described features.

Emacs commands generally involve the @code{CONTROL} key (sometimes
labeled @code{CTRL} or @code{CTL}) or the @code{META} key.  On some
keyboards, the @code{META} key is labeled @code{ALT} or @code{EDIT} or
something else (for example, on Sun keyboards, the diamond key to the
left of the space-bar is @code{META}).  If there is no @code{META} key,
the @code{ESC} key can be used, instead.  Rather than writing out
@code{META} or
@code{CONTROL} each time we want to prefix a character, we will use the
following abbreviations:

@multitable @columnfractions .2 .8
@item @code{C-<chr>}
@tab means hold the @key{CONTROL} key while typing the character
@code{<chr>}. Thus, @code{C-f} would be: hold the @key{CONTROL} key and
type @code{f}.
@item @code{M-<chr>}
@tab means hold the @key{META} key down while typing @code{<chr>}.  If there
is no @key{META} key, type @key{ESC}, release it, then type the
character @code{<chr>}.
@end multitable

For users new to Emacs, we highly recommend that they go through the
interactive Emacs tutorial: type @code{C-h t} to start it.

For others, it is important to understand the following Emacs concepts:
@table @asis
@item window
In Emacs terminology, a window refers to separate panes within the same
window of the window system, and not to overlapping, separate
windows. When using @sc{Singular} within Emacs, extra windows may appear
which display help or output from certain commands. The most important
window commands are:
@multitable @columnfractions .1 .2 .6
@item @code{C-x 1}
@tab @code{File->Un-Split}
@tab Un-Split window (i.e., kill other windows)
@item @code{C-x o}
@tab
@tab Goto other window, i.e. move cursor into other window.
@end multitable

@item cursor and point
The location of the cursor in the text is also called "point".  To
paraphrase, the cursor shows on the screen where point is located in
the text. Here is a summary of simple cursor-moving operations:
@multitable @columnfractions .2 .8
@item @code{C-f} @tab   Move forward a character
@item @code{C-b} @tab   Move backward a character
@item @code{M-f} @tab   Move forward a word
@item @code{M-b} @tab   Move backward a word
@item @code{C-a} @tab   Move to the beginning of line
@item @code{C-e} @tab   Move to the end of line
@end multitable

@item buffer
Any text you see in an Emacs window is always part of some buffer. For
example, each file you are editing with Emacs is stored inside a buffer,
but also @sc{Singular} is running inside an Emacs buffer. Each buffer
has a name: for example, the buffer of a file you edit usually has the
same name as the file, @sc{Singular} is running in a buffer which has
the name @code{*singular*} (or, @code{*singular<2>*},
@code{*singular<3>*}, etc., if you have multiple @sc{Singular}
sessions within the same Emacs).

When you are asked for input to an Emacs command, the cursor
moves to the bottom line of Emacs, i.e., to a special buffer, called the
"minibuffer". Typing @key{RETURN} within the minibuffer, ends the
input, typing @key{SPACE} within the minibuffer, lists all possible
input values to the interactive Emacs command.

The most important buffer commands are
@multitable @columnfractions .2 .8
@item @code{C-x b} @tab Switch buffer
@item @code{C-x k} @tab Kill current buffer
@end multitable
Alternatively, you can switch to or kill buffers using the @code{Buffer}
menu.

@item Executing commands
Emacs commands are executed by typing @code{M-x <command-name>}
(remember that @key{SPACE} completes partial
command names). Important and frequently used commands have short-cuts
for their execution: Key bindings or even menu entries. For example, a
file can be loaded with @code{M-x load-file}, or @code{C-x C-f}, or with
the @code{File->Open} menu.

@item How to exit
To end the Emacs (and, @sc{Singular}) session,
type @code{C-x C-c} (two characters), or use the @code{File -> Exit}
menu.

@item When Emacs hangs
If Emacs stops responding to your commands, you can stop it safely by
typing @code{C-g}, or, if this fails, by typing @code{C-]}.

@item More help
Nearly all aspects of Emacs are very well documented: type
@code{C-h}  and then a character saying what kind of help you want.
For example, typing @code{C-h i} enters the @code{Info} documentation
browser.

@item Using the mouse
Emacs is fully integrated with the mouse. In particular, clicking the
right mouse button brings up a pop-up menu which usually contains a few
commonly used commands.
@end table

@c --------------------------------------------------------------------------
@node Running SINGULAR under Emacs, Demo mode, A quick guide to Emacs, Emacs user interface
@subsection Running SINGULAR under Emacs
@cindex Emacs, running Singular under
@cindex Singular, running within Emacs
@cindex Running SINGULAR under Emacs

There are two ways to start the @sc{Singular} Emacs interface: Typing
@code{ESingular} instead of @code{Singular} on the command shell
launches a new Emacs process, initializes the interface and runs
@sc{Singular} within Emacs. The other way is to start the interface in
an already running Emacs, by typing @code{M-x singular} inside
Emacs. This initializes the interface and runs @sc{Singular} within
Emacs. Both ways are described in more detail below.

Note: To properly run the Emacs interface, several files are needed
which usually reside in the @code{emacs} subdirectory of your
@sc{Singular} distribution. This directory is called
singular-emacs-home-directory in the following.

@subheading Starting the interface using ESingular

As mentioned above, @code{ESingular} is an "out-of-the-box" solution:
You don't have to add special things to your @code{.emacs} startup file
to initialize the interface; everything is done for you in a special
file called @code{.emacs-singular} (which comes along with the
@sc{Singular} distribution and resides in the
singular-emacs-home-directory) which is automatically loaded on Emacs
startup (and the loading of the @code{.emacs} file is automatically
suppressed).

The customizable variables of the @sc{Singular} Emacs interface are set
to defaults which give the novice user a very shell like
feeling of the interface. Nevertheless, these default settings can be
changed, see @ref{Customization of the Emacs interface}. Besides other
Emacs initializations, such as fontification or blinking parentheses, a
new menu item called @code{Singular} is added to the main menu,
providing menu items for starting @sc{Singular}. On XEmacs, a button
starting @sc{Singular} is added to the main toolbar.

The @sc{Singular} interface is started automatically; once you see a
buffer called @code{*singular*} and the @sc{Singular} prompt, you are
ready to start your @sc{Singular} session.

@code{ESingular} inherits all @code{Singular} options. For a description
of all these options, see @ref{Command line options}. Additionally
there are the following options which are special to @code{ESingular}:

@multitable @columnfractions .3 .7

@item command-line option /
@*environment variable
@tab functionality

@item @code{--emacs=EMACS}
@*@code{ESINGULAR_EMACS}
@tab Use @code{EMACS} as Emacs program to run the @sc{Singular} Emacs
interface, where @code{EMACS} may e.g. be emacs or xemacs.

@item @code{--emacs-dir=DIR}
@*@code{ESINGULAR_EMACS_DIR}
@tab Set the singular-emacs-home-directory, which is the directory where
singular.el can be found,  to @code{DIR}.

@item @code{--emacs-load=FILE}
@*@code{ESINGULAR_EMACS_LOAD}
@tab Load @code{FILE} on Emacs start-up, instead of the default load file.

@item @code{--singular=PROG}
@*@code{ESINGULAR_SINGULAR}
@tab Start @code{PROG} as @sc{Singular} program within Emacs

@end multitable

Notice that values of these options can also be given by setting the
above mentioned environment variables (where values  given as
command-line arguments take priority over values given by environment
variables).

@subheading Starting the interface within a running Emacs

If you are a more experienced Emacs user and you already have your own
local @code{.emacs} startup file, you might want to start the interface
out of your running Emacs without using @code{ESingular}. For this, you
should add the following lisp code to your @code{.emacs} file:

@smallexample
(setq load-path (cons "<singular-emacs-home-directory>" load-path))
(autoload 'singular "singular"
  "Start Singular using default values." t)
(autoload 'singular-other "singular"
  "Ask for arguments and start Singular." t)
@end smallexample

Then typing @code{M-x singular} in a running Emacs session initializes
the interface in a new buffer and launches a @sc{Singular} process. The
@sc{Singular} prompt comes up and you are ready to start your
@sc{Singular} session.

It is a good idea to take a look at the (well documented) file
@code{.emacs-singular} in the singular-emacs-home-directory, which comes
along with the distribution. In it you find some useful initializations
of the @sc{Singular} interface as well as some lisp code, which, for
example, adds a button to the XEmacs toolbar. Some of this code might be
useful for your @code{.emacs} file, too. And if you are an Emacs wizard,
it is of course a good idea to take a look at @code{singular.el} in the
singular-emacs-home-directory.

@subheading CYGWIN and ESingular
@cindex CYGWIN and ESingular
@cindex ESingular, CYGWIN
@itemize
@item @strong{X11 server}
install @code{xlaunch}, @code{emacs-X11}, @code{xterm} and all dependencies.
Create with @code{xlaunch} a startup file for the X-server which also starts
the client @code{xterm}. From that one can start ESingular.
@item @strong{fork problems}
The simplest way to overcome fork problem is to run @code{/usr/bin/rebase-trigger full},
then stop all Cygwin processes and services, and then run @code{setup-x86.exe}.
The _autorebase postinstall script will then take care of the rebase.
Occasionally it is necessary to reboot the computer before doing this.
@end itemize
@subheading Starting, interrupting and stopping SINGULAR

There are the following commands to start and stop @sc{Singular}:

@itemize @bullet
@item @code{singular-other} (or menu @code{Singular}, item
@code{Start...})

Starts a @sc{Singular} process and asks for the following four
parameters in the minibuffer area:

@enumerate
@item
The @sc{Singular} executable. This can either be a file name with complete
path, e.g., @code{/local/bin/Singular}. Then exactly this executable is
started. The path may contain the character @code{~} denoting your home
directory. Or it can be the name of a command without path, e.g.,
@code{Singular}. Then the executable is searched for in your
@code{$PATH} environment variable.

@item
The default working directory. This is the path to an existing
directory, e.g., @code{~/work}. The current directory is set to this
directory before @sc{Singular} is started.

@item
Command line options. You can set any @sc{Singular} command line
option (see @ref{Command line options}).

@item
The buffer name. You can specify the name of the buffer the interface is
running in.

@end enumerate

@item @code{singular} (or menu @code{Singular}, item @code{Start
default})

Starts @sc{Singular} with default settings for the executable, the
working directory, command line switches, and the buffer name. You can
customize this default settings, see
@ref{Customization of the Emacs interface}.

@item @code{singular-exit-singular} (bound to @code{C-c $} or menu
@code{Singular}, item @code{Exit})

Kills the running @sc{Singular} process of the current
buffer (but does not kill the buffer). Once you have killed a
@sc{Singular} process you can start a new one in the same buffer with
the command @code{singular} (or select the item @code{Start default} of
the @code{Singular} menu).

@item @code{singular-restart} (bound to @code{C-c C-r} or menu
@code{Singular}, item @code{Restart})

Kills the running @sc{Singular} process of the current buffer and starts
a new process in the same buffer with exactly the same command line
arguments as before.

@item @code{singular-control-c} (bound to @code{C-c C-c} or menu @code{Singular}, item @code{Interrupt})

Interrupt the @sc{Singular} process running in the current buffer. Asks
whether to (@code{a})bort the current @sc{Singular} command,
(@code{q})uit or (@code{r})estart the current @sc{Singular} process, or
(@code{c})ontinue without doing anything (default).
@end itemize

Whenever a @sc{Singular} process is started within the Emacs interface,
the contents of a special startup file (by default
@code{~/.emacs-singularrc}) is pasted as input to @sc{Singular} at the
very end of the usual startup sequence (see @ref{Startup sequence}). The
name of the startup file can be changed, see
@ref{Customization of the Emacs interface}.


@c --------------------------------------------------------------------------
@node Demo mode, Customization of the Emacs interface, Running SINGULAR under Emacs, Emacs user interface
@subsection Demo mode
@cindex Emacs, Singular demo mode
@cindex Singular, demo mode
@cindex Demo mode

The Emacs interface can be used to run interactive @sc{Singular}
demonstrations. A demonstration is started by loading a so-called
@sc{Singular} demo file with the Emacs command
@code{singular-demo-load}, bound to
@code{C-c C-d}, or with the menu @code{Commands->Load Demo}.

A @sc{Singular} demo file should consist of @sc{Singular} commands
separated by blank lines. When running a demo, the input up to the next
blank line is echoed to the screen. Hitting @key{RETURN} executes the
echoed commands and shows their output. Hitting @key{RETURN} again,
echos the next commands to the screen, and so on, until all commands of
the demo file are executed. While running a demo, you can
execute other commands on the @sc{Singular} prompt: the next input
from the demo file is then echoed again, if you hit @key{RETURN} on
an empty input line.

A @sc{Singular} demo can prematurely be exited by either starting
another demo, or by executing the Emacs command
@code{singular-demo-exit} (menu: @code{Commands->Exit Demo}).

Some aspects of running @sc{Singular} demos can be customized.
@xref{Customization of the Emacs interface}, for more info.


@c --------------------------------------------------------------------------
@node Customization of the Emacs interface, Editing SINGULAR input files with Emacs, Demo mode, Emacs user interface
@subsection Customization of the Emacs interface
@cindex Emacs, customization of Singular mode
@cindex Singular, customization of Emacs user interface
@cindex Customization of the Emacs interface

Emacs provides a convenient interface to customize the behavior of
Emacs and the SINGULAR Emacs interface for your own needs. You enter the
customize environment by either calling @code{M-x customize} (on XEmacs
you afterwards have to enter @code{emacs} in the minibuffer area) or by
selecting the menu item @code{Options->Customize->Emacs...} for XEmacs,
and the menu item @code{Help->Customize->Toplevel Customization Group}
for Emacs, resp. A brief introduction to the customization mode comes up
with the customization buffer. All customizable parameters are
hierarchically grouped and you can browse through all these groups and
change the values of the parameters using the mouse. At the end you can
safe your settings to a file making your changes permanent.

To change the settings of the @sc{Singular} Emacs interface you can
either select the item @code{Preferences} of the @code{Singular} menu,
call @code{M-x customize-group} and give the argument
@code{singular-interactive} in the minibuffer area, or browse from the
top-level customization group through the path
@code{External->Singular->Singular interactive}.

The @sc{Singular} interface customization buffer is divided into four
groups:

@itemize @bullet
@item Singular Faces

Here you can specify various faces used if font-lock-mode is enabled
(which, by default, is).

@item Singular Sections And Foldings

Here you can specify special faces for @sc{Singular} input and output
and change the text used as replacement for folded sections.

For doing this, you also might
find handy the function @code{customize-face-at-point},
which lets you customize the face at the current position of
point. This function is automatically defined if you run
@code{ESingular}). Otherwise, you should add its definition (see below)
to your personal @code{.emacs} file.

@item Singular Interactive Miscellaneous

Here you can specify various things such as the behavior of
the cursor keys, the name of the special @sc{Singular} startup file, the
appearance of the help window, or the default values for the
@code{singular} command.
@c Time: I find this sentence rather confusing
@c For a first look at customization of the
@c interface, this is probably the best group.


@item Singular Demo Mode

Here you can specify how chunks of the demo file are divided, or specify
a default directory for demo files.
@end itemize

When you run @code{ESingular}, the settings of customized variables are
saved in the file @code{$HOME/.emacs-singular-cust}. Otherwise, the
settings are appended to your @code{.emacs} file. Among others, this
means that the customized settings of @code{ESingular} are not
automatically  taken over by a "normal" Emacs, and vice versa.


@c --------------------------------------------------------------------------
@node Editing SINGULAR input files with Emacs, Top 20 Emacs commands, Customization of the Emacs interface, Emacs user interface
@subsection Editing SINGULAR input files with Emacs
@cindex Emacs, editing Singular input files
@cindex Singular, editing input files with Emacs
@cindex Editing SINGULAR input files with Emacs

Since @sc{Singular}'s programming language is similar to C, you
should use the Emacs C/C++-mode to edit @sc{Singular} input files and
@sc{Singular} libraries. Among others, this Emacs mode provides
automatic indentation, line-breaking and keyword highlighting.

When running @code{ESingular}, the C/C++-mode is automatically turned on
whenever a file with the suffix @code{.sing}, or @code{.lib} is loaded.

For Emacs sessions which were not started by @code{ESingular},  you
should add the following to your @code{.emacs} file:
@smallexample
;; turn on c++-mode for files ending in ".sing" and ".lib"
(setq auto-mode-alist (cons '("\\.sing\\'" . c++-mode) auto-mode-alist))
(setq auto-mode-alist (cons '("\\.lib\\'" .  c++-mode) auto-mode-alist))
;; turn-on fontification for c++-mode
(add-hook 'c++-mode-hook
         (function (lambda () (font-lock-mode 1))))
;; turn on aut-new line and hungry-delete
(add-hook 'c++-mode-hook
          (function (lambda () (c-toggle-auto-hungry-state 1))))
;; a handy function for customization
(defun customize-face-at-point ()
  "Customize face which point is at."
  (interactive)
  (let ((face (get-text-property (point) 'face)))
    (if face
        (customize-face face)
      (message "No face defined at point"))))

@end smallexample

Notice that you can change the default settings for source-code
highlighting (colors, fonts, etc.) by customizing the respective faces
using the @code{Customize} feature of Emacs. For doing this, you might
find handy the above given function @code{customize-face-at-point},
which lets you customize the face of the current position of point (this
function is automatically defined if you run @code{ESingular}).

@c --------------------------------------------------------------------------
@node Top 20 Emacs commands,,Editing SINGULAR input files with Emacs, Emacs user interface
@subsection Top 20 Emacs commands
@cindex Emacs, important commands
@cindex Singular, important commands of Emacs interface
@cindex Top 20 Emacs commands

Here is a list of the 20 probably most useful commands when using the
@sc{Singular} Emacs interface.

Starting and stopping of @sc{Singular}:

@itemize @bullet

@item @code{singular} (menu @code{Singular->Start Default...}): starts
@sc{Singular} using default arguments.

@item @code{singular-other} (menu @code{Singular->Start}): starts
@sc{Singular} asking for several arguments in the minibuffer area.

@item @code{singular-exit} (key @code{C-c $} or menu
@code{Singular->Exit}): kills the @sc{Singular} process running in the
current buffer (but does not kill the buffer).

@item @code{singular-restart} (key @code{C-c C-r} or menu
@code{Singular->Restart}): kills the @sc{Singular} process running in
the current buffer and starts a new @sc{Singular} process with exactly
the same arguments as before.

@end itemize

Editing input and output:

@itemize @bullet

@item @code{singular-beginning-of-line} (key @code{C-a}): moves point to
beginning of line, then skips past the @sc{Singular} prompt, if any.

@item @code{singular-toggle-truncate-lines} (key @code{C-c C-t} or menu
@code{Commands->Truncate lines}): toggles whether long lines should be
truncated or not. If lines are not truncated, the commands
@code{singular-scroll-left} and @code{singular-scroll-right} are useful
to scroll left and right, resp.

@item @code{singular-dynamic-complete} (key @code{TAB}): performs
context specific completion. If point is inside a string, file name
completion is done. If point is at the end of a help command (i.e.,
@code{help} or @code{?}), completion on @sc{Singular} help topics is
done. If point is at the end of an example command (i.e.,
@code{example}), completion is done on @sc{Singular} examples. In all
other cases, completion on @sc{Singular} commands is done.

@c note: add space between Fold/ Unfold to avoid overfull line
@item @code{singular-folding-toggle-fold-latest-output} (key @code{C-c
C-o} or menu @code{Commands->Fold/ Unfold Latest Output}): toggles
folding of the latest output section. If your last @sc{Singular} command
produced a huge output, simply type @code{C-c C-o} and it will be
replaced by a single line.

@item @code{singular-folding-toggle-fold-at-point} (key @code{C-c C-f}
or menu @code{Commands->Fold/Unfold At Point}): toggles folding of the
section the point currently is in.

@item @code{singular-folding-fold-all-output} (menu @code{Commands->Fold
All Output}): folds all @sc{Singular} output, replacing each output
section by a single line.

@item @code{singular-folding-unfold-all-output} (menu
@code{Commands->Unfold All Output}): unfolds all @sc{Singular} output
sections showing their true contents.

@end itemize

Loading of files and @sc{Singular} demo mode:

@itemize @bullet

@item @code{singular-load-library} (key @code{C-c C-l} or menu
@code{Commands->Libraries->other...}): asks for a standard library name
or a library file in the minibuffer (hit @code{TAB} for completion) and
loads the library into @sc{Singular}. The submenu @code{Libraries} of
the @code{Commands} menu also provides a separate menu item for each
standard library.

@item @code{singular-load-file} (key @code{C-c <} or menu
@code{Commands->Load File...}): asks for a file name in the minibuffer
(which is expanded using @code{expand-file-name} if given a prefix
argument) and loads the file into @sc{Singular}.

@item @code{singular-demo-load} (key @code{C-c C-d} or menu
@code{Commands->Load Demo...}): asks for a file name of a @sc{Singular}
demo file in the minibuffer area (hit @code{SPACE} for completion) and
enters the @sc{Singular} demo mode showing the first chunk of the demo.

@item @code{singular-demo-exit} (menu @code{Commands->Exit Demo}):
exits from @sc{Singular} demo mode and cleans up everything that is
left from the demo.

@end itemize

Help and Customization:

@itemize @bullet

@item @code{singular-help} (key @code{C-h C-s} or menu
@code{Singular->Singular Help}): asks for a @sc{Singular} help topic in
the minibuffer (hit @code{TAB} for completion) and shows the help text
in a separate buffer.

@item @code{singular-example} (key @code{C-c C-e} or menu
@code{Singular->Singular Example}): asks for a @sc{Singular} command in
the minibuffer (hit @code{TAB} for completion) and executes the example
of this command in the current @sc{Singular} buffer.

@item @code{customize-group} (menu @code{Singular->Preferences}): enters
the customization group of the @sc{Singular} Emacs interface. (If called
via @code{M-x customize-group} give argument @code{singular-interactive}
in the minibuffer area.)

@end itemize


@c ---------------------------------------------------------------------------
@c ---------------------------------------------------------------------------
@node Rings and orderings, Implemented algorithms, Emacs user interface, General concepts
@section Rings and orderings
@cindex Rings and orderings
@cindex QQ
@cindex ZZ
@cindex ZZ/p
@cindex ZZ/m

All non-trivial algorithms in @sc{Singular} require the prior definition of
a ring. Such a ring can be

@enumerate
@item
a polynomial ring over a field,
@item
a polynomial ring over a ring
@item
a localization of 1.
@item
a quotient ring by an ideal of 1.@: or 2.,
@item
a tensor product of 1.@: or 2.
@end enumerate

Except for quotient rings, all of these rings are realized by choosing a
coefficient field, ring variables, and an appropriate global or local
monomial ordering on the ring variables.  @xref{Term orderings},
@ref{Mathematical background}.

The coefficient field of the rings may be

@cindex real
@cindex complex
@enumerate
@item
the field of rational numbers @math{Q} (@code{QQ}),
@item
@tex
finite fields $Z/p$, $p$ a prime $\le 2147483647$,
@end tex
@ifinfo
finite fields Z/p, p a prime <= 2147483647 (@code{(ZZ/p)}),
@end ifinfo
@item
@tex
finite fields $\hbox{GF}(p^n)$ with $p^n$ elements, $p$ a prime, $p^n \le 2^{16}$,
@end tex
@ifinfo
finite fields GF(p^n) with p^n elements, p a prime, p^n <= 2^16,
@end ifinfo
@item
transcendental extension of @math{Q} or @math{Z/p},
@item
simple algebraic extension of @math{Q} or @math{Z/p},
@item
the field of real numbers represented by floating point
numbers of a user defined precision,
@item
the field of complex numbers represented by (pairs of) floating point
numbers of a user defined precision,
@item
the ring of integers (@code{ZZ}),
@item
finite rings @math{Z/m} with @math{m\in Z}.
@end enumerate

In case of coefficient rings, which are not fields, only the following functions
are guaranteed to work:

@itemize
@item
- basic polynomial arithmetic, i.e. addition, multiplication, division, exponentiation
@item
- std, i.e. computing standard bases (and related: syz, etc.)
@item
- interred
@item
- reduce
@end itemize

@cindex basering
Throughout this manual, the current active ring in @sc{Singular} is
called basering.  The reserved name @code{basering} in @sc{Singular} is
an alias for the current active ring.  The basering can be set by
declaring a new ring as described in the following subsections or by using
the commands @code{setring} and @code{keepring}. @xref{keepring}, @ref{setring}.

Objects of ring dependent types are local to a ring. To access them
after a change of the basering they have to be mapped using @code{map} or by
the functions @code{imap} or @code{fetch}.  @xref{Objects}, @ref{fetch},
@ref{imap}, @ref{map}.

All changes of the basering in a procedure are local to this procedure
unless a @code{keepring} command is used as the last statement of the
procedure.  @xref{Procedures}, @ref{keepring}.

@menu
* Examples of ring declarations::
* General syntax of a ring declaration::
* Term orderings::
* Coefficient rings::
@end menu

@c --------------------------------------
@node Examples of ring declarations, General syntax of a ring declaration, Rings and orderings, Rings and orderings
@subsection Examples of ring declarations
@cindex Examples of ring declarations

The exact syntax of a ring declaration is given in the next two
subsections; this subsection lists some examples first. Note that the chosen
ordering implies that a unit-elements of the ring will be among the elements
with leading monomial 1. For more information, see @ref{Monomial orderings}.

Every floating point number in a ring consists of two parts, which may be
chosen by the user. The leading part represents the number and the rest
is for numerical stability. Two numbers with a difference only in the
rest will be regarded equal.

@itemize @bullet
@item
the ring @math{Z/32003[x,y,z]} with degree reverse lexicographical
ordering.  The exact ring declaration may be omitted in the first
example since this is the default ring:

@smallexample
ring r1;
ring r2 = 32003,(x,y,z),dp;
ring r3=(ZZ/32003)[x,y,z];
ring r4 = (ZZ/32003),(x,y,z),dp;
@end smallexample

@item
similar examples with indexed variables. The ring variables of
r1 are going to be x(1)..x(10); in r2 they will be x(1)(1),
x(1)(2), ..., x(1)(8), x(2)(1), ..., x(5)(8):

@smallexample
ring r1 = 32003,(x(1..10)),dp;
ring r2 = 32003,(x(1..5)(1..8)),dp;
ring r3 = (ZZ/32003)[x(1..5)(1..8)];
ring r4 = (ZZ/32003),(x(1..5)(1..8)),dp;
@end smallexample

@item
the ring @math{Q[a,b,c,d]} with lexicographical ordering:

@smallexample
ring r1 = 0,(a,b,c,d),lp;
ring r2 = QQ,(a,b,c,d),lp;
@end smallexample

@item
the ring @math{Z/7[x,y,z]} with local degree reverse lexicographical
ordering.  The non-prime 10 is converted to the next lower prime in the
second example:

@smallexample
ring r1 = 7,(x,y,z),ds;
ring r2 = 10,(x,y,z),ds;
ring r3 = (ZZ/7),(x,y,z),ds;
@end smallexample

@item
the ring
@tex
$Z/7[x_1,\ldots,x_6]$
@end tex
@ifinfo
Z/7[x1,@dots{},x6]
@end ifinfo
with lexicographical ordering for
@tex
$x_1,x_2,x_3$
@end tex
@ifinfo
x1,x2,x3
@end ifinfo
and degree reverse lexicographical ordering for
@tex
$x_4,x_5,x_6$:
@end tex
@ifinfo
x4,x5,x6:
@end ifinfo

@smallexample
ring r1 = 7,(x(1..6)),(lp(3),dp);
ring r2 = (ZZ/7),(x(1..6)),(lp(3),dp);
@end smallexample

@item
the localization of @math{(Q[a,b,c])[x,y,z]} at the maximal ideal
@math{(x,y,z)}:

@smallexample
ring r1 = 0,(x,y,z,a,b,c),(ds(3), dp(3));
ring r2 = QQ,(x,y,z,a,b,c),(ds(3), dp(3));
@end smallexample

@item
the ring @math{Q[x,y,z]} with weighted reverse lexicographical ordering.
The variables @math{x}, @math{y}, and @math{z} have the weights 2, 1,
and 3, respectively, and  vectors are first ordered by components (in
descending order) and then by monomials:
@smallexample
ring r1 = 0,(x,y,z),(c,wp(2,1,3));
ring r2 = QQ,(x,y,z),(c,wp(2,1,3));
@end smallexample
For ascending component order, the component ordering @code{C} has to be
used.

@item
the ring @math{K[x,y,z]}, where @math{K=Z/7(a,b,c)} denotes the transcendental
extension of @math{Z/7} by @math{a}, @math{b} and @math{c} with degree
lexicographical ordering:

@smallexample
ring r = (7,a,b,c),(x,y,z),Dp;
@end smallexample

@item
the ring @math{K[x,y,z]}, where @math{K=Z/7[a]} denotes the algebraic extension of
degree 2 of @math{Z/7} by @math{a.} In other words, @math{K} is the finite field with
49 elements.  In the first case, @math{a} denotes an algebraic
element over @math{Z/7} with minimal polynomial
@tex
$\mu_a=a^2+a+3$,
@end tex
@ifinfo
mu_a=a^2+a+3,
@end ifinfo
in the second case, @math{a}
refers to some generator of the cyclic group of units of @math{K}:

@smallexample
ring r = (7,a),(x,y,z),dp; minpoly = a^2+a+3;
ring r = (7^2,a),(x,y,z),dp;
@end smallexample

@item
the ring @math{R[x,y,z]}, where @math{R} denotes the field of real
numbers represented by simple precision floating point numbers. This is
a special case:

@smallexample
ring r = real,(x,y,z),dp;
@end smallexample

@item
the ring @math{R[x,y,z]}, where @math{R} denotes the field of real
numbers represented by floating point numbers of 50 valid decimal digits
and the same number of digits for the rest:

@smallexample
ring r = (real,50),(x,y,z),dp;
@end smallexample

@item
the ring @math{R[x,y,z]}, where @math{R} denotes the field of real
numbers represented by floating point numbers of 10 valid decimal digits
and with 50 digits for the rest:

@smallexample
ring r = (real,10,50),(x,y,z),dp;
@end smallexample

@item
the ring @math{R(j)[x,y,z]}, where @math{R} denotes the field of real
numbers represented by floating point numbers of 30 valid decimal digits
and the same number for the rest. @math{j} denotes the imaginary unit.

@smallexample
ring r = (complex,30,j),(x,y,z),dp;
@end smallexample

@item
the ring @math{R(i)[x,y,z]}, where @math{R} denotes the field of real
numbers represented by floating point numbers of 6 valid decimal digits
and the same number for the rest. @math{i} is the default for the imaginary unit.

@smallexample
ring r = complex,(x,y,z),dp;
@end smallexample

@item
the quotient ring @math{Z/7[x,y,z]} modulo the square of the maximal
ideal @math{(x,y,z)}:

@smallexample
ring R = 7,(x,y,z), dp;
qring r = std(maxideal(2));
@end smallexample

@item
the ring @math{Z[x,y,z]}:

@smallexample
ring R = integer,(x,y,z), dp;
@end smallexample

@item
the ring @math{Z/6^3[x,y,z]}:

@smallexample
ring R = (integer, 6, 3),(x,y,z), dp;
@end smallexample

@item
the ring @math{Z/100[x,y,z]}:

@smallexample
ring R = (integer, 100),(x,y,z), dp;
@end smallexample

@end itemize

@c ---------------------------------------------------------------------------
@node General syntax of a ring declaration, Term orderings, Examples of ring declarations, Rings and orderings
@subsection General syntax of a ring declaration
@cindex General syntax of a ring declaration

@subsubheading Rings
@c ------------------------------------------------------------
@c   This piece of text (partially) exists also in the
@c   file types.doc, chapter "ring declarations".
@c   If you change something here, change it there, too!
@c ------------------------------------------------------------
@table @strong
@item Syntax:
@code{ring} name @code{=} @code{(}coefficients@code{),}
  @code{(} names_of_ring_variables @code{),}
  @code{(} ordering @code{);}
or@*
@code{ring} name @code{=} cring
  @code{[} names_of_ring_variables @code{]}
@item Default:
@code{(ZZ/32003)[x,y,z];}
@item Purpose:
declares a ring and sets it as the current basering.
The second form sets the ordering to @code{(dp,C)}.
@code{cring} stands currently for @code{QQ} (the rationals), @code{ZZ} (the integers)
or @code{(ZZ/m)} (the field (m prime and <2147483648) resp. ring of the integers modulo m).
@end table

The coefficients (for the first form) are given by one of the following:
@enumerate
@item
a @code{cring} as given above
@item
a non-negative int_expression less than 2147483648 (2^31).
@* The int_expression should either be 0, specifying the field of
rational numbers Q, or a prime number p, specifying the finite field
with p elements.  If it is not a prime number, int_expression is
converted to the next lower prime number.

@item
an expression_list of an int_expression and one or more names.
@* The int_expression specifies the characteristic of the coefficient
field as described above. The names are used as parameters in
transcendental or algebraic extensions of the coefficient
field. Algebraic extensions are implemented for one parameter only. In
this case, a minimal polynomial has to be defined by an assignment to
@code{minpoly}. @xref{minpoly}.

@item
an expression_list of an int_expression and a name.
@* The int_expression has to be a prime number p to the power of a
positive integer n. This defines the Galois field
@tex
$\hbox{GF}(p^n)$ with $p^n$ elements, where $p^n$ has to be less than or equal to $2^{15}$.
@end tex
@ifinfo
GF(p^n) with p^n elements, where p^n has to be smaller or equal 2^15.
@end ifinfo
The given name refers to a primitive element of
@tex
$\hbox{GF}(p^n)$
@end tex
@ifinfo
GF(p^n)
@end ifinfo
generating the multiplicative group.  Due to a different internal
representation, the arithmetic operations in these coefficient fields
are faster than arithmetic operations in algebraic extensions as
described above.

@item
an expression_list of the name @code{real} and two optional int_expressions
determining the precision in decimal digits and
the size for the stabilizing rest. The default for the rest is the same size
as for the representation.
An exception is the name @code{real} without any integers.
These numbers are implemented as machine floating point numbers
of single precision.
Note that computations over all these fields are not exact.

@item
an expression_list of the name @code{complex},
two optional int_expression and a name.
This specifies the field of complex numbers represented by floating point
numbers with a precision similar to @code{real}. An expression_list
without int_expression defines a precision and rest with length 6.
The name of the imaginary unit is given by the last parameter.
Note that computations over these fields are not exact.

@item
an expression_list with the name @code{integer}. This specifies the ring of integers.

@item
an expression_list with the name @code{integer} and one subsequent int_expression.
This specifies the ring of integers modulo the given int_expression.

@item
an expression_list with the name @code{integer} and two int_expressions @code{b} and @code{e}.
This specifies the ring of integers modulo b^e. If @code{b = 2} and @code{e < int_bit_size}
an optimized implementation is used.

@end enumerate

'names_of_ring_variables' is a list of names or indexed names.

'ordering' is a list of block orderings where each block ordering is either

@enumerate
@item
@code{lp}, @code{dp}, @code{Dp}, @code{ls}, @code{ds}, or @code{Ds}
optionally followed by a size parameter in parentheses.

@item
@code{wp}, @code{Wp}, @code{ws}, @code{Ws}, or @code{a} followed by a
weight vector given as an intvec_expression in parentheses.

@item
@code{M} followed by an intmat_expression in parentheses.

@item
@code{c} or @code{C}.
@end enumerate

For the definition of the orderings, see @ref{Monomial orderings}.

If one of coefficients, names_of_ring_variables, and ordering
consists of only one entry, the parentheses around this entry may be
omitted.
@c ------------------------------------------------------------
@c   End of duplicate piece of text.
@c ------------------------------------------------------------

@subsubheading Quotient rings
@c ------------------------------------------------------------
@c   This piece of text (partially) exists also in the
@c   file types.doc, chapter "qring declarations", and in
@c   singular.doc, chapter "Miscellaneous oddities".
@c   If you change something here, change it there, too!
@c ------------------------------------------------------------
@table @strong
@item Syntax:
@code{qring} name @code{=} ideal_expression @code{;}
@item Default:
none
@item Purpose:
declares a quotient ring as the basering modulo ideal_expression, and sets
it as current basering.
@end table

ideal_expression has to be represented by a standard basis.

The most convenient way to map objects from a ring to its quotient ring
and vice versa is to use the @code{fetch} function (@pxref{fetch}).

@sc{Singular} computes in a quotient ring as long as possible with the
given representative of a polynomial, say, @code{f}.  I.e., it usually
does not reduce @code{f} w.r.t. the quotient ideal.  This is only done
when necessary
during standard bases computations or by an explicit reduction using
the command @code{reduce(f, std(0))} (@pxref{reduce}).

Operations based on standard bases (e.g. @code{std},@code{groebner}, etc., @code{reduce}) and functions which require a standard basis (e.g. @code{dim},@code{hilb}, etc.) operated with the residue classes; all others on the polynomial objects.

@strong{Example:}
@smallexample
@c example
  // definition and usage:
  ring r=(ZZ/32003)[x,y];
  poly f=x3+yx2+3y+4;
  qring q=std(maxideal(2));
  basering;
  poly g=fetch(r, f);
  g;
  reduce(g,std(0));
  // polynomial and residue class:
  ring R=QQ[x,y];
  qring Q=std(y);
  poly p1=x;
  poly p2=x+y;
  // comparing polynomial objects:
  p1==p2;
  // comparing residue classes:
  reduce(p1,std(0))==reduce(p2,std(0));
@c example
@end smallexample

@c ------------------------------------------------------------
@c   End of duplicate piece of text.
@c ------------------------------------------------------------

@c ---------------------------------------------------------------------------
@node Term orderings,  Coefficient rings, General syntax of a ring declaration, Rings and orderings
@subsection Term orderings
@cindex Monomial orderings, Term orderings

Any polynomial (resp.@: vector) in @sc{Singular} is ordered w.r.t. a
term ordering (or, monomial ordering), which has to be specified
together with the declaration of a ring. @sc{Singular} stores and
displays a polynomial (resp.@: vector) w.r.t. this ordering, i.e., the
greatest monomial (also called the leading monomial) is the first one
appearing in the output polynomial, and the smallest monomial is the last one.

@strong{Remark:} The novice user should generally use the ordering
@code{dp} for computations in the polynomial ring
@tex
$K[x_1,\ldots,x_n]$,
@end tex
@ifinfo
K[x1,...,xn],
@end ifinfo
resp.@:  @code{ds} for computations in the localization
@tex
$\hbox{Loc}_{(x)}K[x_1,\ldots,x_n])$.
@end tex
@ifinfo
Loc_(x)K[x1,...,xn]).
@end ifinfo
For more details, see @ref{Polynomial data}.

In a ring declaration, @sc{Singular} offers the following orderings
(but see also @ref{Monomial orderings}):
@enumerate
@item Global orderings
@table @asis
@item @code{lp}
lexicographical ordering

@item @code{rp}
reverse lexicographical ordering, i.e. a lexicographical ordering from the right
with 1 < x_1 < ... <x_n
(should not be used as it reverses the "natural" x_1 > ... > x_n,
reorder the variables instead)

@item @code{dp}
degree reverse lexicographical ordering

@item @code{Dp}
degree lexicographical ordering

@item @code{wp(} intvec_expression @code{)}
weighted reverse lexicographical ordering; the weight vector is expected to
consist of positive integers only.

@item @code{Wp(} intvec_expression @code{)}
weighted lexicographical ordering; the weight vector is expected to consist of
positive integers only.
@end table

Global orderings are well-orderings, i.e., @math{1 < x} for each ring
variable @math{x}. They are denoted by a @code{p} as the second
character in their name.

@item Local orderings
@table @asis
@item @code{ls}
negative lexicographical ordering

@item @code{rs}
negative reverse lexicographical ordering, i.e. a lexicographical ordering from the right
(should not be used as it reverses the "natural" x_1 < ... < x_n,
reorder the variables instead)

@item @code{ds}
negative degree reverse lexicographical ordering

@item @code{Ds}
negative degree lexicographical ordering

@item @code{ws(} intvec_expression @code{)}
(general) weighted reverse lexicographical ordering; the first element
of the weight vector has to be non-zero.

@item @code{Ws(} intvec_expression @code{)}
(general) weighted lexicographical ordering; the first element
of the weight vector has to be non-zero.
@end table

Local orderings are not well-orderings. They are denoted by an @code{s}
as the second character in their name.

@item Matrix orderings
@table @asis
@item @code{M(} intmat_expression @code{)}
intmat_expression has to be an invertible square matrix
@end table

Using matrix orderings, @sc{Singular} can compute standard bases
w.r.t.@: any monomial ordering which is compatible with the natural
semi-group structure on the monomials. In practice, the predefined global
and local orderings together with the block orderings should be
sufficient in most cases. These orderings are faster than their
corresponding matrix orderings since evaluation of a matrix ordering is more
time consuming.

@item Extra weight vector
@table @asis
@item @code{a(} intvec_expression @code{)}
an extra weight vector @code{a(} intvec_expression @code{)} may precede
any monomial ordering
@end table

@item Product ordering
@table @asis
@item @code{(} ordering [ @code{(} int_expression @code{)} ]@code{,} @dots{} @code{)}
any of the above orderings and the extra weight vector may be combined
to yield product or block orderings
@end table

The orderings @code{lp}, @code{dp}, @code{Dp}, @code{ls}, @code{ds},
@code{Ds} and @code{rp} may be followed by an int_expression in
parentheses giving the
size of the block. For the last block the size is calculated
automatically. For weighted orderings, the size of the block is given
by the size of the weight vector. The same holds analogously for matrix
orderings.

@item Module orderings
@table @asis
@item @code{(} ordering@code{,} @dots{}@code{, C )}
@itemx @code{(} ordering@code{,} @dots{}@code{, c )}
sort polynomial vectors by the monomial ordering first, then by
components

@item @code{( C,} ordering@code{,} @dots{} @code{)}
@itemx @code{( c,} ordering@code{,} @dots{} @code{)}
sort polynomial vectors by components first, then by the monomial
ordering
@end table

Here a capital @code{C} sorts generators in ascending order, i.e.,
@code{gen(1)} < @code{gen(2)} < @enddots{} A small @code{c} sorts in
descending order, i.e., @code{gen(1)} > @code{gen(2)} > @enddots{} It is
not necessary to specify the module ordering explicitly since @code{(}
ordering@code{,} @dots{}@code{, C )} is the default.

In fact, @code{c} or @code{C} may be specified anywhere in a product
ordering specification, not only at its beginning or end.  All monomial
block orderings preceding the component ordering have higher precedence,
all monomial block orderings following after it have lower precedence.
@end enumerate

For a mathematical description of these orderings, see
@ref{Polynomial data}.

@c ---------------------------------------------------------------------------
@node Coefficient rings,  , Term orderings, Rings and orderings
@subsection Coefficient rings
@cindex coefficient rings, ring of integers, zero divisors, p-adic numbers

@sc{Singular} supports coefficient ranges which are not fields, i.e. the
integers @math{Z} and the finite rings @math{Z/n} for a number @code{n}. These
coefficient rings were implemented in @sc{Singular} 3.0.5 and at the moment
only limited functionality is available.

@subheading p-adic numbers
@cindex p-adic numbers
@cindex l-adic numbers
@cindex projective limes

The p-adic integers @math{Z_p} are the projective limit of the finite
rings @math{Z/p^n} for @code{n} to infinity. Therefore, computations in this ring
can be approximated by computations in @math{Z/p^n} for large @code{n}.

@c ---------------------------------------------------------------------------
@node Implemented algorithms, The SINGULAR language, Rings and orderings, General concepts
@section Implemented algorithms
@cindex Implemented algorithms

The basic algorithm in @sc{Singular} is a general standard basis algorithm for
any monomial ordering which is compatible with the natural semi-group
structure of the exponents. This includes well-orderings
(Buchberger algorithm to compute a Groebner basis)
and tangent cone orderings (Mora algorithm) as special cases.

Nonetheless, there are a lot of other important algorithms:
@itemize @bullet

@item
Algorithms to compute the standard operations on ideals and modules:
intersection, ideal quotient, elimination, etc.

@item
Different Syzygy algorithms and algorithms to compute free resolutions
of modules.

@item
Combinatorial algorithms to compute dimensions, Hilbert series,
multiplicities, etc.

@item
Algorithms for univariate and multivariate polynomial factorization,
resultant and gcd computations.
@end itemize

@c ----------------------------------------------------------------------
@subheading Commands to compute standard bases
@table @code
@item facstd
@ref{facstd}
@*computes a list of Groebner bases via the Factorizing Groebner Basis
Algorithm, i.e., their intersection has the same radical as the original ideal. It need not
be a Groebner basis of the given ideal.

The intersection of the zero-sets is the zero-set of the given ideal.
@item fglm
@ref{fglm}
@*computes a Groebner basis provided that a reduced Groebner basis
w.r.t.@: another ordering is given.

Implements the so-called FGLM (Faugere, Gianni, Lazard, Mora) algorithm.
The given ideal must be zero-dimensional.
@item groebner
@ref{groebner}@*
computes a standard resp.@: Groebner basis using a heuristically chosen
method.

This is the preferred method to compute a standard resp.@: Groebner
bases.
@item mstd
@ref{mstd}
@*computes a standard basis and a minimal set of generators.
@item std
@ref{std}
@*computes a standard resp.@: Groebner basis.
@item stdfglm
@ref{stdfglm}
@*computes a Groebner basis in a ring with a ``difficult'' ordering
(e.g., lexicographical) via @code{std} w.r.t.@: a ``simple'' ordering and
@code{fglm}.

The given ideal must be zero-dimensional.
@item stdhilb
@ref{stdhilb}@*
computes a Groebner basis in a ring with a ``difficult'' ordering
(e.g., lexicographical) via @code{std} w.r.t.@: a ``simple'' ordering
and a @code{std} computation guided by the Hilbert series.
@end table

@c ----------------------------------------------------------------------
@subheading Further processing of standard bases
The next commands require the input to be a standard basis.

@table @code
@item degree
@ref{degree}
@*computes the (Krull) dimension, codimension and the multiplicity.

The result is only displayed on the screen.
@item dim
@ref{dim}@*
computes the dimension of the ideal resp.@: module.
@item highcorner
@ref{highcorner}@*
computes the smallest monomial not contained in the ideal resp.@: module.
The ideal resp.@: module has to be finite dimensional as a vector space
over the ground field.
@item hilb
@ref{hilb}@*
computes the first, and resp.@: or, second Hilbert series of an ideal
resp.@: module.
@item kbase
@ref{kbase}@* computes a vector space basis (consisting of monomials) of
the quotient of a ring by an ideal resp.@: of a free module by a
submodule.

The ideal resp.@: module has to be finite dimensional as a vector space
over the ground field and has to be represented by a standard basis w.r.t.@:
the ring ordering.
@item mult
@ref{mult}@*
computes the degree of the monomial ideal resp.@: module generated by
the leading monomials of the input.
@item reduce
@ref{reduce}@*
reduces a polynomial, vector, ideal or module to its normal form with
respect to an ideal or module represented by a standard basis.
@item vdim
@ref{vdim}@*
computes the vector space dimension of a ring (resp.@: free module)
modulo an ideal (resp.@: module).
@end table

@c ----------------------------------------------------------------------
@subheading Commands to compute resolutions
@table @code
@item res
@ref{res}@*
computes a free resolution of an ideal or module using a heuristically
chosen method.
This is the preferred method to compute free resolutions of ideals or
modules.
@item fres
@ref{fres}@*
improved version of @ref{sres}, computes a free resolution of an ideal or
module using Schreyer's method. The input has to be a standard basis.
@item lres
@ref{lres}@*
computes a free resolution of an ideal or module with LaScala's
method. The input needs to be homogeneous.
@item mres
@ref{mres}@*
computes a minimal free resolution of an ideal or module with the Syzygy
method.
@item sres
@ref{sres}@*
computes a free resolution of an ideal or module with Schreyer's
method. The input has to be a standard basis.
@item nres
@ref{nres}@*
computes a free resolution of an ideal or module with the standard basis
method.
@item syz
@ref{syz}@*
computes the first Syzygy (i.e., the module of relations of the
given generators).
@end table

@c ----------------------------------------------------------------------
@subheading Further processing of resolutions
@table @code
@item betti
@ref{betti}@*
computes the graded Betti numbers of a module from a free resolution.
@item minres
@ref{minres}@*
minimizes a free resolution of an ideal or module.
@item regularity
@ref{regularity}@*
computes the regularity of a homogeneous ideal resp.@: module from a given
minimal free resolution.
@end table

@c ----------------------------------------------------------------------
@subheading Processing of polynomials
@table @code
@item char_series
@ref{char_series}@*
computes characteristic sets of polynomial ideals.
@item extgcd
@ref{extgcd}@*
computes the extended gcd of two polynomials.

This is implemented as extended Euclidean Algorithm, and applicable for univariate
polynomials only.
@item factorize
@ref{factorize}@*
computes factorization of univariate and multivariate polynomials into
irreducible factors.

The most basic algorithm is univariate factorization in prime
characteristic.  The Cantor-Zassenhaus Algorithm is used in this case.
For characteristic 0, a univariate Hensel-lifting is done to lift from
prime characteristic to characteristic 0.  For multivariate
factorization in any characteristic, the problem is reduced to the
univariate case first, then a multivariate Hensel-lifting is used to
lift the univariate factorization.

Factorization of polynomials over algebraic extensions is provided by
factoring the norm for univariate polynomials f (the gcd of f and the
factors of the norm is a factorization of f) resp. by the extended Zassenhaus
algorithm for multivariate polynomials.

@item gcd
@ref{gcd}@*
computes greatest common divisors of univariate and multivariate polynomials.

In the univariate case NTL is used.
For prime characteristic, a subresultant gcd is used.  In characteristic 0,
the EZGCD is used, except for a special case where a modular algorithm is used.

@item resultant
@ref{resultant}@*
computes the resultant of two univariate polynomials using the subresultant
algorithm.

Multivariate polynomials are considered as univariate polynomials in the
main variable (which has to be specified by the user).
@item vandermonde
@ref{vandermonde}@*
interpolates a polynomial from its values at several points
@end table

@c ----------------------------------------------------------------------
@subheading Matrix computations
@table @code
@item bareiss
@ref{bareiss}@*
implements sparse Gauss-Bareiss method for elimination (matrix
triangularization) in arbitrary integral domains.
@item det
@ref{det}@*
computes the determinant of a square matrix.

For matrices with integer entries a modular algorithm is used.  For
other domains the Gauss-Bareiss method is used.
@item minor
@ref{minor}@*
computes all minors (=subdeterminants) of a given size for a matrix.
@end table

@c ----------------------------------------------------------------------
@subheading Numeric computations
@table @code
@item laguerre
@ref{laguerre}@*
computes all (complex) roots of a univariate polynomial
@item uressolve
@ref{uressolve}@*
finds all roots of a 0-dimensional ideal with multivariate resultants
@end table

@c ----------------------------------------------------------------------
@subheading Controlling computations
@table @code
@item option
@ref{option}@*
allows setting of options for manipulating the behaviour of computations
(such as reduction strategies) and for showing protocol information indicating
the progress of a computation.
@end table

@c ---------------------------------------------------------------------------
@node The SINGULAR language, Input and output, Implemented algorithms, General concepts
@section The SINGULAR language
@cindex The SINGULAR language

@sc{Singular} interprets commands given interactively on the command
line as well as given in the context of user-defined procedures.  In
fact, @sc{Singular} makes no distinction between these two cases.
Thus, @sc{Singular} offers a powerful programming language as well as
an easy-to-use command line interface without differences in syntax or
semantics.

In the following, the basic language concepts such as commands,
expressions, names, objects, etc., are discussed.  @xref{Procedures},
and @ref{Libraries},
for the concepts of procedures
and libraries.

In many aspects, the @sc{Singular} language is similar to the C
programming language.  For a description of some of the subtle
differences, see @ref{Major differences to the C programming language}.

@subsubheading Elements of the language

The major building blocks of the @sc{Singular} language  are
expressions, commands, and control structures.  The notion of expressions
in the @sc{Singular} and the C programming language are identical,
whereas the notion of commands and control structures only roughly
corresponds to C statements.

@itemize @bullet

@item
An ``expression'' is a sequence of operators, functions, and operands
that specifies a computation.  An expression always results in a
value of a specific type.  @xref{Data types}, and its subsections (e.g.,
@ref{poly expressions}), for information on how to build expressions.

@item
A ``command'' is either a declaration, an assignment, a call to a
function without return value, or a print command.  For detailed
information, see @ref{General command syntax}.

@item
``Control structures'' determine the execution sequence of commands.
@sc{Singular} provides control structures for conditional execution
(@code{if} @dots{} @code{else}) and iteration (@code{for} and
@code{while}).  Commands may be grouped in pairs of @code{@{} @code{@}}
(curly brackets) to form blocks.  @xref{Control structures}, for
more information.
@end itemize

@subsubheading Other notational conventions
For user-defined functions, the notions of ``procedure'' and
``function'' are synonymous.

As already mentioned above, functions without return values are called
commands.  Furthermore, whenever convenient, the term ``command'' is
used for a function, even if it does return a value.

@menu
* General command syntax::
* Special characters::
* Names::
* Objects::
* Type conversion and casting::
* Flow control::
@end menu

@c ---------------------------------------------------------------------------
@node General command syntax, Special characters, The SINGULAR language, The SINGULAR language
@subsection General command syntax
@cindex General command syntax
@cindex alias

In @sc{Singular} a command is either a declaration, an assignment, a
call to a function without return value, or a print command. The general
form of a command is described in the following subsections.

@subsubheading Declaration
@enumerate
@item type name @code{=} expression @code{;}
@* declares a variable with the given name of the given type and assigns
the expression as initial value to it.  Expression is an expression of
the specified type or one that can be converted to that type.
@xref{Type conversion and casting}.

@item @code{alias} type name
@*Introduces name as an alternative, read-only name for another variable_name.
Can only be used in procedure headings to avoid copying large data.

@item type name_list @code{=} expression_list @code{;}
@* declares variables with the given names and assigns successively each
expression of expression_list to the corresponding name of
name_list.  Both lists must be of the same length.  Each expression in
expression_list is an expression of the specified type or one that can
be converted to that type.  @xref{Type conversion and casting}.

@item type name @code{;}
@* declares a variable with the given name of the given type and assigns
the default value of the specific type to it.
@end enumerate

@xref{Names}, for more information on declarations.  @xref{Data types},
for a description of all data types known to @sc{Singular}.

@smallexample
ring r;                   // the default ring
poly f,g = x^2+y^3,xy+z2; // the polynomials f=x^2+y^3 and g=x*y+z^2
ideal I = f,g;            // the ideal generated by f and g
matrix m[3][3];           // a 3 x 3 zero matrix
int i=2;                  // the integer i=2
@end smallexample

@subsubheading Assignment
@enumerate 4
@item
name @code{=} expression @code{;}
@* assigns expression to name.

@item
name_list @code{=} expression_list @code{;}
@* assigns successively each expression of expression_list to the
corresponding name of name_list. Both lists must be of the same
length.  This is not a simultaneous assignment.  Thus, @code{f, g = g, f;} does
not swap the values of @code{f} and @code{g}, but rather assigns
@code{g} to both @code{f} and @code{g}.
@end enumerate

A type conversion of the type of expression to the type of
name must be possible. @xref{Type conversion and casting}.

An assignment itself does not yield a value. Hence, compound assignments
like @code{i = j = k;} are not allowed and result in an error.

@smallexample
f = x^2 + y^2 ;      // overrides the old value of f
I = jacob(f);
f,g = I[1],x^2+y^2 ; // overrides the old values of f and g
@end smallexample

@subsubheading Function without return value
@enumerate 6
@item
function_name [ @code{(} argument_list @code{)} ] @code{;}
@* calls function function_name with arguments argument_list.
@end enumerate

The function may have output (not to be confused with a return value of
type string). @xref{Functions}. Functions without a return value are
specified there to have a return type 'none'.

Some of these functions have to be called without parentheses, e.g.,
@code{help}, @code{LIB}.

@smallexample
@c example
  ring r;
  ideal i=x2+y2,x;
  i=std(i);
  degree(i);        // degree has no return value but prints output
@c example
@end smallexample

@subsubheading Print command
@enumerate 7
@item
expression @code{;}
@* prints the value of an expression, for example, of a variable.
@end enumerate

Use the function @code{print} (or the procedure @code{show} from inout.lib)
to get a pretty output of various data
types, e.g., matrix or intmat. @xref{print}.

@smallexample
@c example
  int i=2;
  i;
  intmat m[2][2]=1,7,10,0;
  print(m);
@c example
@end smallexample

@c ------------------------------------------------------------
@node Special characters, Names, General command syntax, The SINGULAR language
@subsection Special characters
@cindex Special characters
The following characters and operators have special meanings:
@table @asis
@item @code{=}
@cindex =
assignment
@item @code{@{}, @code{@}}
@cindex @{
@cindex @}
parentheses for block programming
@item @code{(}, @code{)}
@cindex (
@cindex )
@cindex indexed names
in expressions, for indexed names and for argument lists
@item @code{[}, @code{]}
@cindex [
@cindex ]
access operator for strings, integer vectors, ideals, matrices,
polynomials, resolutions, and lists.  Used to build vectors of
polynomials.  Example: @code{s[3]}, @code{m[1,3]}, @code{i[1..3]},
@code{[f,g+x,0,0,1]}.
@item @code{+}
@cindex +
addition operator
@item @code{++}
@cindex ++
increment operator
@item @code{-}
@cindex -
subtraction operator
@item @code{--}
@cindex --
decrement operator
@item @code{*}
@cindex *
multiplication operator
@item @code{/}
@cindex /
division  operator.  @xref{Miscellaneous oddities}, for the difference
between the division operators @code{/} and @code{div}.
@item @code{%}
@cindex %
modulo operator (@code{mod} is an alias to @code{%}): result is always non-negative
@item @code{^} or @code{**}
@cindex ^
@cindex **
exponentiation operator
@item @code{==}
@cindex ==
comparison operator equal
@item @code{!=} or @code{<>}
@cindex !=
@cindex <>
comparison operator not equal
@item @code{>=}
@cindex >=
comparison operator larger than or equal to
@item @code{>}
@cindex >
comparison operator larger
@item @code{<=}
@cindex <=
comparison operator smaller than or equal to
@item @code{<}
@cindex <
comparison operator smaller. Also used for file input. @xref{filecmd}.
@item @code{!}
@cindex !
boolean operator not
@item @code{&&}
@cindex &&
boolean operator and
@item @code{||}
@cindex ||
boolean operator or
@item @code{"}
@cindex "
delimiter for string constants
@item @code{`}
@cindex `
delimiter for name substitution
@item @code{?}
@cindex ?
synonym for @code{help}
@item @code{//}
@cindex //
@cindex comment
comment delimiter.  Comment extends to the end of the line.
@item @code{/*}
@cindex //
comment delimiter.  Starts a comment which ends with @code{*/}.
@item @code{*/}
@cindex //
comment delimiter.  Ends a comment which starts with @code{/*}.
@item @code{;}
@cindex ;
statement separator
@item @code{,}
separator for expression lists and function arguments
@item @code{\}
@cindex \
escape character for @code{"} and @code{\} within strings
@item @code{..}
@cindex ..
interval specifier returning intvec.
E.g., @code{1..3} which is equivalent to the intvec @code{1, 2, 3}.
@item @code{:}
@cindex :
repeated entry.
E.g., @code{3:5} generates an intvec of length 5 with constant entries 3, i.e., (3, 3, 3, 3, 3).
@item @code{::}
@cindex ::
accessor for package members.
E.g., @code{MyPackage::i} accesses variable @code{i} in package @code{MyPackage}.
@item @code{_}
@cindex _
value of expression displayed last
@item @code{~}
@cindex ~
breakpoint in procedures
@item @code{#}
@cindex #
list of parameters in procedures without explicit parameter list
@item @code{$}
@cindex $
terminates @sc{Singular}
@end table

@c ------------------------------------------------------------
@node Names, Objects, Special characters, The SINGULAR language
@subsection Names
@cindex Names
@cindex Names, indexed
@cindex indexed names
@cindex Variables, indexed
@cindex multi indices
@cindex indices, multi
@cindex Identifiers, syntax of

@sc{Singular} is a strongly typed language.  This means that all names
(= identifiers) have to be declared prior to their use.  For the general
syntax of a declaration, see the description of declaration commands
(@pxref{General command syntax}).

@xref{Data types}, for a description of @sc{Singular}'s data types.
@xref{typeof}, for a short overview of possible types.  To get information
on a name and the object named by it, the @code{type} command may be
used (@pxref{type}).

It is possible to redefine an already existing name if doing so does not
change its type. A redefinition first sets the variable to the default
value and then computes the expression. The difference between
redefining and overriding a variable is shown in the following example:

@smallexample
@c example
  int i=3;
  i=i+1;        // overriding
  i;
  int i=i+1;    // redefinition
  i;
@c example
@end smallexample

User defined names should start with a letter and consist of letters and
digits only.  As an exception to this rule, the characters @code{@@},
and @code{_} may
be used as part of a name, too (@code{@@} as the first letter is reserved
for purposes of library routines). Capital and small letters are
distinguished.  Indexed names are built as a name followed by an
int_expression in parentheses.  A list of indexed names can be built as
a name followed by an intvec_expression in parentheses.
For multi-indices, append an int_expression in parentheses to an
indexed name.
An alternative multi-index construction is
@code{name_prefix(} index_1, index_2,... @code{)} where the @code{name_prefix}
must be an undefined name.

@smallexample
@c example
  ring R;
  int n=3;
  ideal j(3);
  ideal j(n);     // is equivalent to the above
  ideal j(2)=x;
  j(2..3);
  ring r=0,(x(1..2)(1..3)(1..2)),dp;
  r;
  int i(1,2),i(2,3);
  i(2,3);
@c example
@end smallexample

Names must not coincide with reserved names (keywords).  Type
@code{reservedName();} to get a list of the reserved names.
@xref{reservedName}.  Names should not interfere with names of ring
variables or, more generally, with monomials.  @xref{Identifier
resolution}.
@* The command @code{listvar} provides a list of the names in use
(@pxref{listvar}).

The most recently printed expression is available
under the special name @code{_}, e.g.,
@smallexample
@c example
  ring r;
  ideal i=x2+y3,y3+z4;
  std(i);
  ideal k=_;
  k*k+x;
  size(_[3]);
@c example
@end smallexample

A string_expression enclosed in @code{`}@dots{}@code{`} (back ticks)
evaluates to the value of the variable given by the string_expression.
This feature is referred to as name substitution.

@smallexample
@c example
  int foo(1)=42;
  string bar="foo";
  `bar+"(1)"`;
@c example
@end smallexample

@c ------------------------------------------------------------
@node Objects, Type conversion and casting, Names, The SINGULAR language
@subsection Objects
@cindex Objects

Every object in @sc{Singular} has a type and a value.  In most cases it
has also a name and in some cases an attribute list.  The value of an
object may be examined simply by printing it with a print command:
object@code{;}.  The type of an object may be determined by means of the
@code{typeof} function, the attributes by means of the @code{attrib}
function (@ref{typeof}, @ref{attrib}):

@smallexample
@c example
  ring r=0,x,dp;
  typeof(10);
  typeof(10000000000000000);
  typeof(r);
  attrib(x);
  attrib(std(ideal(x)));
@c example
@end smallexample

Each object of type
@code{poly},
@code{ideal},
@code{vector},
@code{module},
@code{map},
@code{matrix},
@code{number}, or
@code{resolution}
belongs to a specific ring.  This is also true for @code{list}, if at least one of the
objects contained in the list belongs to a ring.  These objects are local
to the ring.  Their names can be duplicated for other objects in other rings.
Objects from one ring can be mapped to another ring using maps or the
commands @code{fetch} or @code{imap}.  @xref{map}, @ref{fetch}, @ref{imap}.

All other types do not belong to a ring and can be accessed within every
ring and across rings.  They can be declared even if there is no active
basering.

@c ------------------------------------------------------------
@node Type conversion and casting, Flow control, Objects, The SINGULAR language
@subsection Type conversion and casting

@subsubheading Type conversion
@cindex Type conversion
Assignments convert the type of the right-hand side to the type of the
left-hand side of the assignment, if possible.  Operators and functions
which require certain types of operands can also implicitly convert the
type of an expression.  It is, for example, possible to multiply a
polynomial by an integer because the integer is automatically converted to
a polynomial.  Type conversions do not act transitively.  Possible
conversions are:


@c ------------------------------------------------
@c Do not delete the leading `@ ' for indentation of the whole table.  The
@c table is indented since regular @tables are indented by a small amount,
@c too.
@c ------------------------------------------------
@multitable @columnfractions .05 .18 .81
@item
@   1. @tab @code{intvec}  @tab @expansion{} @code{intmat}
@item
@   2. @tab @code{poly}  @tab @expansion{} @code{ideal}
@item
@   3. @tab @code{bigint}  @tab @expansion{} @code{ideal}
@item
@   4. @tab @code{int}  @tab @expansion{} @code{ideal}
@item
@   5. @tab @code{intmat}  @tab @expansion{} @code{matrix}
@item
@   6. @tab @code{ideal}  @tab @expansion{} @code{matrix}
@item
@   7. @tab @code{module}  @tab @expansion{} @code{matrix}
@item
@   8. @tab @code{number}  @tab @expansion{} @code{matrix}
@item
@   9. @tab @code{poly}  @tab @expansion{} @code{matrix}
@item
@   10. @tab @code{vector}  @tab @expansion{} @code{matrix}
@item
@   11. @tab @code{bigint}  @tab @expansion{} @code{matrix}
@item
@   12. @tab @code{int}  @tab @expansion{} @code{matrix}
@item
@   13. @tab @code{intvec}  @tab @expansion{} @code{matrix}
@item
@   14. @tab @code{ideal}  @tab @expansion{} @code{module}
@item
@   15. @tab @code{matrix}  @tab @expansion{} @code{module}
@item
@   16. @tab @code{vector}  @tab @expansion{} @code{module}
@item
@   17. @tab @code{bigint}  @tab @expansion{} @code{number}
@item
@   18. @tab @code{int}  @tab @expansion{} @code{number}
@item
@   19. @tab @code{number}  @tab @expansion{} @code{poly}
@item
@   20. @tab @code{bigint}  @tab @expansion{} @code{poly}
@item
@   21. @tab @code{int}  @tab @expansion{} @code{poly}
@item
@   22. @tab @code{list}  @tab @expansion{} @code{resolution}
@item
@   23. @tab @code{poly}  @tab @expansion{} @code{vector}
(p @expansion{} p*@code{gen(1)})
@item
@   24. @tab @code{bigint}  @tab @expansion{} @code{vector}
@item
@   25. @tab @code{int}  @tab @expansion{} @code{vector}
(i @expansion{} i*@code{gen(1)})
@item
@   26. @tab @code{int}  @tab @expansion{} @code{bigint}
@item
@   27. @tab @code{int}  @tab @expansion{} @code{intvec}
@item
@   28. @tab @code{string}  @tab @expansion{} @code{link}
@item
@   29. @tab @code{resolution}  @tab @expansion{} @code{list}
@end multitable

@subsubheading Type casting
@cindex Type casting
An expression can be casted to another type by using a type cast
expression:
@* type @code{(} expression @code{)}.

Possible type casts are:

@c ------------------------------------------------
@c Do not delete the leading `@ ' for indentation of the whole table.  The
@c table is indented since regular @tables are indented by a small amount,
@c too.
@c ------------------------------------------------
@multitable @columnfractions .05 .15 .83
@item
@    @tab to @tab from
@item
@    @tab  @code{bigint} @tab  expression @code{int}, @code{number}, @code{poly}
@item
@    @tab  @code{ideal} @tab  expression lists of @code{int}, @code{number}, @code{poly}
@item
@    @tab  @code{ideal} @tab  @code{int}, @code{matrix}, @code{module}, @code{number}, @code{poly}, @code{vector}
@item
@    @tab  @code{int} @tab    @code{number}, @code{poly}
@item
@    @tab  @code{intvec} @tab expression lists of @code{int}, @code{intmat}
@item
@    @tab  @code{intmat} @tab @code{intvec} (@pxref{intmat type cast})
@item
@    @tab  @code{list} @tab   expression lists of any type
@item
@    @tab  @code{matrix} @tab @code{module}, @code{ideal},
@code{vector}, @code{matrix}.
@* There are two forms to convert something to a matrix: if @code{matrix(}
expression @code{)} is used then the size of the matrix is determined
by the size of expression.
@* But @code{matrix(} expression @code{,} m @code{,} n @code{)} may also be
used - the result is a
@tex
$ m \times n $
@end tex
@ifinfo
m x n
@end ifinfo
matrix (@pxref{matrix type cast})
@item
@    @tab  @code{module} @tab expression lists of @code{int}, @code{number},
@code{poly}, @code{vector}
@item
@    @tab  @code{module} @tab @code{ideal}, @code{matrix}, @code{vector}
@item
@    @tab  @code{number} @tab @code{poly}
@item
@   @tab  @code{poly} @tab   @code{int}, @code{number}
@item
@    @tab  @code{ring} @tab   @code{list} (the inverse of @code{ringlist})
@item
@   @tab  @code{string} @tab   any type (@pxref{string type cast})
@end multitable

@strong{Example:}
@smallexample
@c example
  ring r=0,x,(c,dp);
  number(3x);
  number(poly(3));
  ideal i=1,2,3,4,5,6;
  print(matrix(i));
  print(matrix(i,3,2));
  vector v=[1,2];
  print(matrix(v));
  module(matrix(i,3,2));
  // generators are columns of a matrix
@c example
@end smallexample


@c ------------------------------------------------------------
@node Flow control, ,Type conversion and casting, The SINGULAR language
@subsection Flow control
@cindex Flow control
@cindex block

A block is a sequence of commands surrounded by @{ and @}.
@smallexample
@{
   command;
   @dots{}
@}
@end smallexample
Blocks are used whenever @sc{Singular} is used as a structured programming
language. The @code{if} and @code{else} structures allow conditional
execution of blocks (see @ref{if}, @ref{else}). @code{for} and @code{while}
loops are available for a repeated execution of blocks (see @ref{for},
@ref{while}). In procedure definitions, the main part and the example section
are blocks as well(see @ref{proc}).
@menu
* break::
* quit::
* keepring::
* export::
* exportto::
* if::
* importfrom::
* else::
* for::
* while::
* proc::
* return::
* continue::
@end menu


@c ----------------------------------------------------------------------
@c ----------------------------------------------------------------------
@node Input and output, Procedures, The SINGULAR language, General concepts
@section Input and output
@cindex input
@cindex output

@sc{Singular}'s input and output (short, I/O) are realized using links.
Links are the communication channels of @sc{Singular}, i.e., something
@sc{Singular} can write to and read from.  In this section, a short
overview of the usage of links and of the different link types is given.

For loading of libraries, see @ref{LIB}. For executing program scripts,
see @ref{filecmd}.

@subheading Monitoring
A special form of I/O is monitoring.  When monitoring is enabled,
@sc{Singular} makes a typescript of everything printed on your terminal
to a file.  This is
useful to create a protocol of a @sc{Singular} session.  The
@code{monitor} command enables and disables this feature
(@pxref{monitor}).


@subheading How to use links
Recall that links are the communication channels of @sc{Singular}, i.e.,
something @sc{Singular} can write to and read from using the functions
@code{write} and @code{read}. There are furthermore
the functions @code{dump} and @code{getdump} which store resp.@:
retrieve the content of an entire @sc{Singular} session to, resp.@: from,
a link.  The @code{dump} and @code{getdump} commands are not available
for DBM links.

For more information, see @ref{write}, @ref{read}, @ref{dump},
@ref{getdump}.

@strong{Example:}
@smallexample
@c example unix_only
  ring r; poly p = x+y;
  dump(":w test.sv");   // dump the session to the file test.sv
  kill r;               // kill the basering
  listvar();            // no output after killing the ring
  getdump(":r test.sv");// read the dump from the file
  listvar();
@c example
@end smallexample

Specifying a link can be as easy as specifying a filename as a string.
Links do not even need to be explicitly opened
or closed before, resp.@: after, they are used.  To explicitly open or
close a link, the @code{open}, resp.@: @code{close}, commands may be
used (see @ref{open}, @ref{close}).

Links have various properties which can be queried using the
@code{status} function (@pxref{status}).

@strong{Example:}
@smallexample
@c example unix_only
  link l = "ssi:fork";
  l;
  open(l);
  status(l, "open");
  close(l);
  status(l, "open");
@c example
@end smallexample

@subheading ASCII links
Data that can be converted to a string can be written into files for
storage or communication with other programs. The data are written in
plain ASCII format. Reading from an ASCII link returns a string ---
conversion into other data is up to the user. This can be done, for
example, using the command @code{execute} (@pxref{execute}).

ASCII links should primarily be used for storing small amounts of data,
especially if it might become necessary to manually inspect or
manipulate the data.

@xref{ASCII links}, for more information.

@strong{Example:}
@smallexample
@c example
  // (over)write file test.ascii, link is specified as string
  write(":w test.ascii", "int i =", 3, ";");
  // reading simply returns the string
  read("test.ascii");
  // but now test.ascii is "executed"
  execute(read("test.ascii"));
  i;
@c example
@end smallexample

@subheading Ssi links
Data is communicated with other processes (e.g., @sc{Singular}
processes) which may run on the same computer or on different ones.  Data
exchange is accomplished using TCP/IP links in the ssi format.
Reading from an ssi link returns the written expressions (i.e., not a
string, in general).

Ssi links should primarily be used for communicating with other
programs or for parallel computations (see, for example,
@ref{Parallelization with ssi links}).

@xref{Ssi links}, for more information.

@strong{Example:}
@smallexample
@c example
  ring r;
  link l = "ssi:tcp localhost:"+system("Singular"); // declare a link explicitly
  open(l);  // needs an open, launches another SINGULAR as a server
  write(l, x+y);
  kill r;
  def p = read(l);
  typeof(p); p;
  close(l); // shuts down SINGULAR server
@c example
@end smallexample

@subheading DBM links
Data is stored in and accessed from a data base.  Writing is
accomplished by a key and a value and associates the value with the key
in the specified data base.  Reading is accomplished w.r.t.@: a key,
the value associated to it is returned.  Both the key and the value have to
be specified as strings.  Hence, DBM links may be used only for data
which may be converted to or from strings.

DBM links should primarily be used when data needs to be accessed not in a
sequential way (like with files) but in an associative way (like with
data bases).

@xref{DBM links}, for more information.

@strong{Example:}
@smallexample
@c example unix_only
  ring r;
  // associate "x+y" with "mykey"
  write("DBM:w test.dbm", "mykey", string(x+y));
  // get from data base what is stored under "mykey"
  execute(read("DBM: test.dbm", "mykey"));
@c example
@end smallexample


@c ---------------------------------------------------------------------------
@node Procedures, Libraries, Input and output, General concepts
@section Procedures
@cindex Procedures

Procedures contain sequences of commands in the @sc{Singular} language.
They are used to extend the set of commands by user defined commands.
In a @sc{Singular} session, procedures are defined by either typing them
on the command line or by loading them from a library file with the
@code{LIB} or @code{load}  command (@pxref{Libraries}).  A procedure
is invoked like normal built-in commands, i.e., by typing its name
followed by the list of arguments in parentheses. The invocation then
executes the sequence of commands constituting the procedure. All
procedures defined in a @sc{Singular} session can be displayed by
entering @code{listvar(proc);} .

@menu
* Procedure definition::
* Parameter list::
* Help string::
* Names in procedures::
* Procedure-specific commands::
@end menu

See also @xref{Procedures in a library}.
@c -------------------------------------------------
@node Procedure definition, Parameter list,, Procedures
@subsection Procedure definition
@cindex Procedure definition
@cindex procedures, static
@cindex static procedures
@c ------------------------------------------------------------
@c   This piece of text exists also in the file types.doc,
@c   chapter "proc declaration".
@c   If you change something here, change it there, too!
@c ------------------------------------------------------------
@table @strong
@item Syntax:
[@code{static}] @code{proc} proc_name [(<parameter_list>)]
@*[<help_string>]
@*@code{@{}
@*
@tex
\quad
@end tex
   <procedure_body>
@*@code{@}}
@*[@code{example}
@*@code{@{}
@*
@tex
\quad
@end tex
   <sequence_of_commands>
@*@code{@}}]
@item Purpose:
@itemize @bullet
@item
Defines a new function, the @code{proc} proc_name.
@item
The help string, the parameter list, and the example section are optional.
They are, however, mandatory for the procedures listed in the header of a library.
The help string is ignored and no example section is allowed if the procedure is defined
interactively, i.e., if it is not loaded from a file by the @code{LIB} or @code{load}
command (@pxref{LIB} and @pxref{load} ).
@item
Once loaded from a file into a  @sc{Singular} session, the information provided in the help
string will be displayed upon entering @code{help proc_name;}, while the @code{example}
section will be executed  upon entering @code{example proc_name;}.
@xref{Parameter list}, @ref{Help string}, and the example in
@ref{Procedures in a library}.
@item
In the body of a library, each procedure not meant to be accessible by
users should be declared static. @xref{Procedures in a library}.
@end itemize
@end table

@subsubheading Example of an interactive procedure definition and its execution:
@smallexample
@c example
  proc milnor_number (poly p)
  {
    ideal i= std(jacob(p));
    int m_nr=vdim(i);
    if (m_nr<0)
    {
      "// not an isolated singularity";
    }
    return(m_nr);         // the value of m_nr is returned
  }
  ring r1=0,(x,y,z),ds;
  poly p=x^2+y^2+z^5;
  milnor_number(p);
@c example
@end smallexample

@subsubheading Example of a procedure definition in a library:
First, we define the library (and store it as @code{ sample.lib}):
@smallexample
@c include sample.lib
@end smallexample

Now, we load the library and execute its procedures:
@smallexample
@c example error
  LIB "sample.lib";        // load the library sample.lib
  example tab;             // show an example
  "*"+tab(3)+"*";          // use the procedure tab
  // the static procedure internal_tab is not accessible
  "*"+internal_tab(3)+"*";
  // show the help section for tab
  help tab;
@c example
@end smallexample

@c ---------------------------------------
@node Parameter list, Help string, Procedure definition, Procedures
@subsection Parameter list
@cindex Parameter list
@cindex argument, optional
@cindex argument, default
@cindex default argument
@table @code
@item @strong{Syntax:}
@code{( )}
@*@code{(} parameter_definition @code{)}
@item @strong{Purpose:}
@itemize @bullet
@item
Defines the number, type and names of the arguments of a procedure.
@item
The parameter_list is optional.
@item
Adding @code{list #} as argument to a parameter list means to allow
optional parameters. Furthermore, @code{(list #)} is the default for a
parameter list (in case no list is explicitly given). Inside the procedure
body, the arguments of @code{list #}  are referenced by @code{#[1], #[2]},
etc.
@item
If a procedure has optional parameters, the attribute @code{default_arg}
gives the default values for the optional arguments. This provides in
particular the possibility to also change the behaviour of all procedures
nested inside the given procedure.
@end itemize

@item @strong{Example:}
@smallexample
proc x0
@{
    // can be called with
... // any number of arguments of any type: #[1], #[2],...
    // number of arguments: size(#)
@}

proc x1 ()
@{
... // can only be called without arguments
@}

proc x2 (ideal i, int j)
@{
... // can only be called with 2 arguments,
    // which can be converted to ideal resp. int
@}

proc x3 (i,j)
@{
... // can only be called with 2 arguments
    // of any type
    // (i,j) is the same as (def i,def j)
@}

proc x5 (i,list #)
@{
... // can only be called with at least 1 argument
    // number of arguments: size(#)+1
@}

attrib(x5,"default_arg",3);
x5(2); // is equivalent to
x5(2,3);
@end smallexample
@item @strong{Note:}
@* The parameter_list may stretch across multiple lines.
@* A parameter may have any type (including the types @code{proc}
   and @code{ring}).
@*If a parameter is of type ring, then it
   can only be specified by name, but not with a type. For instance:
@smallexample
proc x6 (r)
@{
... // this is correct, r may be of any type, even of type ring
@}

proc x7 (ring r)
@{
... // this is NOT CORRECT
@}
@end smallexample
@end table

@c --------------------------------------- ----------
@node Help string,Names in procedures,Parameter list,Procedures
@subsection Help string
@cindex Help string
@cindex procedures, help string

@table @asis
@item @strong{Syntax:}
  string_constant;
@item @strong{Purpose:}
  Constitutes the help text of a procedure.
@item @strong{Format:}
@smallexample
USAGE:    <proc_name>(<parameter list>);   <explanation of parameters>
ASSUME:  <description of assumptions made>
RETURN:   <description of what is returned>
SIDE EFFECTS:  <description of global objects generated or manipulated,
but not returned>
REMARKS: <information on theory and implemented  algorithms,references>
NOTE:    <particularities, limitations, additional details>
KEYWORDS: <semicolon-separated phrases of index keys>
SEE ALSO: <comma-separated names of related procedures/cross references>
EXAMPLE:  example <proc_name>; shows an example
@end smallexample
@item @strong{NOTE:}
@itemize @bullet
@item
ASSUME, SIDE EFFECTS, KEYWORDS, and SEE ALSO are optional.
No help string is required for static procedures.
@item
EXAMPLE: refers to the example section of the procedure.
In a @sc{Singular} session, the example will be carried out upon
entering @code{example <proc_name>;} if the procedure is loaded
from a file by the @code{LIB} or @code{load}  command (@pxref{LIB}
and @pxref{load} ). No example section is allowed if the procedure
is defined interactively.
@item
See @ref{Typesetting of help and info strings} for help strings
in the @sc{Singular}  documentation.
@item
See the example in @ref{Procedures in a library} for an
illustration.
@end itemize
@end table

@c ---------------------------------------
@node Names in procedures, Procedure-specific commands, Help string, Procedures
@subsection Names in procedures
@cindex Names in procedures
@cindex local names

@itemize @bullet
@item
All variables defined inside a procedure are local to the procedure and their
names cannot interfere with names in other procedures. Without further action,
they are automatically deleted after leaving the procedure.
@item
To keep local variables and their value after leaving the procedure, they have
to be exported (i.e. made global) by a command like @code{export} or
@code{exportto} (@pxref{export},@pxref{exportto}, @pxref{importfrom};
@pxref{package}). To return the value of a local variable, use the
@code{return} command (@pxref{return}).
@end itemize

@strong{Example:}
@smallexample
@c example
proc xxx
{
  int k=4;        //defines a local variable k
  int result=k+2;
  export(result);  //defines the global variable "result".
}
xxx();
listvar(all);
@c example
@end smallexample
Note that the variable @code{result} became a global variable after the
execution of @code{xxx}.


@c --------------------------------------- ----------
@node Procedure-specific commands,, Names in procedures, Procedures
@subsection Procedure-specific commands
@cindex Procedure-specific commands

A few  commands should only be used inside a procedure. They either make
local objects global ones or return results to the level from where the
procedure was called.

@c ref
See
@ref{export};
@ref{exportto};
@ref{return}.
@c ref



@c ----------------------------------------------------------------------
@node Libraries, Debugging tools, Procedures, General concepts
@section Libraries
@cindex Libraries

@itemize @bullet
@item
A library is a collection of @sc{Singular}
procedures in a file.
@item
To load a library into a @sc{Singular} session, use the @code{LIB} or
@code{load} command.
Having loaded a library, its procedures can be used like any built-in
@sc{Singular} function, and information on the library is obtained
by entering @code{help libname.lib;}
@item
@xref{SINGULAR libraries}, for all libraries currently distributed
with  @sc{Singular}.
@item
When writing your own library, it is important to comply with the
guidelines described in this section. Otherwise, due to potential
parser errors, it may not be possible to load the library.
@item
Each library consists of a header and a body. The first line
of a library must start with a double slash @code{//}.
@item
The library header consists of a version string, a category string,
an info string, and LIB commands. The strings are mandatory. LIB
commands are meant to load the additional libraries used by the
library under consideration.
@item
The library body collects the procedures (declared static or not).
@item
No line of a library should consist of more than 60 characters.
@end itemize

@menu
* Libraries in the SINGULAR Documentation::
* Version string::
* Category string::
* Info string::
* LIB commands::
* Procedures in a library::
* template_lib::
* Formal Checker::
* Documentation Tool::
* Typesetting of help and info strings::
* Loading a library::
@end menu

@c -------------------------------------------------------------------
@node Libraries in the SINGULAR Documentation,Version string,Libraries,Libraries
@subsection Libraries in the SINGULAR Documentation
@cindex Libraries in the SINGULAR Documentation

@itemize @bullet
@item
The typesetting language in which the @sc{Singular} documentation
is written is @code{texinfo}. The info string of a library included in the
@sc{Singular} distribution will be parsed and automatically
translated to the @code{texinfo} format. The same applies to the
help string of each procedure listed in the PROCEDURE: section of
the info string.
@item
Based on various tools, @code{info}, @code{dvi}, @code{p}, and
@code{html} versions of the @code{texinfo} documentation are generated.
@item
For texinfo markup elements and other information facilitating optimal
typesetting, see @ref{Typesetting of help and info strings}.
@item
For the convenience of users checking directly the source code, the
@code{texinfo}  tools should be used economically. That is, the info
and  help texts should be well readable verbatim.
@item
The example of each procedure listed in the PROCEDURE: section of the
info string is computed and its output is included in the documentation.
@end itemize


@c -------------------------------------------------------------------
@node Version string,Category string,Libraries in the SINGULAR Documentation,Libraries
@subsection Version string
@cindex Version string

A version string is part of the header of a library.

@table @asis
@item @strong{Syntax:}
  version = string_constant;
@item @strong{Purpose:}
  Defines the version number of a library. It is displayed when the library is loaded.
@item @strong{Example:}
@code{version="version sample.lib 4.0.0.0 Dec_2013 ";}
@item @strong{Note:}
Syntax: @code{version<space><filename><space><version><space><date><space>}
@end table

@c -------------------------------------------------------------------
@node Category string,Info string,Version string,Libraries
@subsection Category string
@cindex Category string

A category string is part of the header of a library.

@table @asis
@item @strong{Syntax:}
  category = string_constant;
@item @strong{Purpose:}
  Defines the category of a library.
@item @strong{Example:}
  category="Algebraic geometry";
@item @strong{Note:}
  Reserved for sorting the libraries into categories.
@end table

@c -------------------------------------------------------------------
@node Info string,LIB commands,Category string,Libraries
@subsection Info string
@cindex Info string


@table @asis
@item @strong{Syntax:}
  info = string_constant;
@item @strong{Purpose:}
  Constitutes the help text of a library. Will be displayed in a
  @sc{Singular}  session upon entering @code{help libname.lib;} .
  Will be part of the @sc{Singular} documentation if the library
  is distributed with @sc{Singular}.
  @xref{Libraries in the SINGULAR  Documentation}.
@item @strong{Format:}
@smallexample
info="
LIBRARY: <library_name> <one line description of the purpose>
AUTHOR:  <name, and email address of author>
OVERVIEW: <concise, additional information on what is implemented>
REFERENCES: <references for further information>
KEYWORDS: <semicolon-separated phrases of index keys>
SEE ALSO: <comma-separated words of cross references>
PROCEDURES:
  <proc_name_1>();     <one line description of the purpose>
     .
     .
  <proc_name_N>();     <one line description of the purpose>
";
@end smallexample

@item @strong{NOTE:}
@itemize @bullet
@item
In the documentation, the one line description of the purpose following
LIBRARY: will be printed in its own line, starting with the prefix PURPOSE: .
@item
REFERENCES, KEYWORDS, and SEE ALSO are optional.
@item
Only non-static procedures should be listed in the PROCEDURES: section.
A procedure parameter should be included between the brackets
@code{()} only if the corresponding one line description of the purpose
refers to it. @xref{Procedures in a library}.
@item
In the documentation,  separate nodes (subsections in printed documents)
are created precisely for those  procedures of the library appearing
n the PROCEDURES: section (that is, for some if not all non-static
procedures of the library).
@end itemize

@item @strong{Example:}
@smallexample
info="
LIBRARY: absfact.lib   Absolute factorization for characteristic 0
AUTHORS: Wolfram Decker,       decker at math.uni-sb.de
         Gregoire Lecerf,      lecerf at math.uvsq.fr
         Gerhard Pfister,      pfister at mathematik.uni-kl.de

OVERVIEW:
A library for computing the absolute factorization of multivariate
polynomials f with coefficients in a field K of characteristic zero.
Using Trager's idea, the implemented algorithm computes an absolutely
irreducible factor by factorizing over some finite extension field L
(which is chosen such that V(f) has a smooth point with coordinates in L).
Then a minimal extension field is determined making use of the
Rothstein-Trager partial fraction decomposition algorithm.

REFERENCES:
G. Cheze, G. Lecerf: Lifting and recombination techniques for absolute
                  factorization. Journal of Complexity, 23(3):380-420, 2007.

KEYWORDS: factorization; absolute factorization.
SEE ALSO: factorize

PROCEDURES:
  absFactorize();        absolute factorization of poly
";
@end smallexample

To see how this infostring appears in the documentation after
typesetting, check @ref{absfact_lib}:
@end table

@c -------------------------------------------------------------------
@node LIB commands,Procedures in a library,Info string,Libraries
@subsection  LIB commands
@cindex  LIB commands


LIB commands are part of the header of a library.

@table @asis
@item @strong{Syntax:}
   LIB "lib_1.lib";@*
   ...                  @*
   LIB "lib_r.lib";
@item @strong{Purpose:}
  Loads libraries used by the library under consideration.
@item @strong{Example:}
@smallexample
    LIB "primdec.lib";
    LIB "normal.lib";
@end smallexample
@item @strong{Note:}
  The keyword LIB must be followed by at least one space.
@end table

@c -------------------------------------------------------------------
@node Procedures in a library,template_lib,LIB commands,Libraries
@subsection  Procedures in a library
@cindex  Procedures in a library

Here asre hints and requirements on how procedures contained in a
library should be implemented. For more on procedures,
see @ref{Procedures}.
@enumerate
@item
Each procedure not meant to be accessible by users should be declared static.
@item
The header of each procedure not declared static must comply with the guidelines
described in @ref{Procedure definition} and @ref{Help string}.
In particular, it must have a help and example section, and assumptions made
should be carefully explained. If the assumptions are checked by the procedure
on run-time, errors may be reported using the @ref{ERROR} function.
@item
Names of procedures should not be shorter than 4 characters and should not
contain any special characters. In particular, the use of @code{_} in names of
procedures is discouraged. If the name of the procedure is composed of
more than one word, each new word should start with a capital letter, all
other letters should be lower case (e.g. linearMapKernel).
@item
No procedures should be defined within the body of another procedure.
@item
A procedure may print out comments, for instance to explain results or to
display intermediate computations. This is often helpful when calling the
procedure directly, but it may also cause confusions in cases where the
procedure is called by another procedure. The @sc{Singular}  solution to
this problem makes use of the function @code{dbprint} (@pxref{dbprint})
and the reserved variables @code{printlevel} and @code{voice}
(@pxref{printlevel} and @pxref{voice}). Note that @code{printlevel} is a
predefined, global variable whose value can be changed by the user, while
@code{voice} is an internal variable, representing the nesting level of
procedures. Accordingly, the value of @ref{voice} is 1 on the top level,
2 inside the first procedure, and so on. The default value of
@code{printlevel}  is 0, but @code{printlevel}  can be set to
any integer value by the user.
@end enumerate

@table @asis
@item @strong{Example:}
If the procedure @code{Test} below is called directly from
the top level, then `comment1' is displayed, but not `comment2'.
By default, nothing is displayed if @code{Test} is called from within any
other procedure. However, if @code{printlevel} is set to a value k
with k>0, then `comment1' (resp. `comment2') is displayed -- provided
@code{Test} is called from another procedure with nesting level at
most k (resp. k-1).

The example part of a procedure behaves in this respect like the
procedure on top level (the nesting level is 1, that is, the value of
@code{voice} is 2). Therefore, due to the command @code{printlevel=1;},
`comment1' will be displayed when entering @code{example Test;}.
However, since printlevel is a global variable, it should be reset to
its old value at the end of the example part.

The predefined variable @code{echo}  controls whether input lines are
echoed or not. Its default is 0, but it can be reset by the user. Input
is echoed if @code{echo>=voice}. At the beginning of the example
part, @code{echo} is set to the value 2. In this way, the input lines
of the example will be displayed when entering @code{example Test;}.

@smallexample
      proc Test
      "USAGE:   ...
               ...
      EXAMPLE: example Test; shows an example
      "
      @{   ...
         int p = printlevel - voice + 3;
          ...
         dbprint(p,"comment1");
         dbprint(p-1,"comment2");
         // dbprint prints only if p > 0
          ...
      @}
      example
      @{ "EXAMPLE:"; echo = 2;
         int p = printlevel;   //store old value of printlevel
         printlevel = 1;       //assign new value to printlevel
          ...
         Test();
         printlevel = p;       //reset printlevel to old value
      @}
@end smallexample

@item @strong{Note:}
@sc{Singular}  functions such as @code{pause} or @code{read}  allow
and require interactive user-input. They are, thus, in particular useful
for debugging purposes. If such a command is used inside the procedure
of a library to be distributed with @sc{Singular}, the example section of
the procedure has to be written with some care -- the procedure should
only be called from within the example if the value of @code{printlevel}
is 0. Otherwise,  the automatic build process of @sc{Singular} will not
run through since the examples are carried out during the build process.
They are, thus, tested against changes in the code.
@end table


@c -------------------------------------------------------------------
@node template_lib,Formal Checker,Procedures in a library,Libraries
@subsection  template_lib
@cindex  template_lib
@cindex Template for writing a library
@cindex library, template

First, we show the source-code of a template library:

@smallexample
@c include template.lib
@end smallexample

Second, we show how the library appears in the documentation
after typesetting (with one subsection for each procedure):

@c lib template.lib  lib_fun lib_ex


@c -------------------------------------------------------------------
@node Formal Checker,Documentation Tool,template_lib,Libraries
@subsection  Formal Checker
@cindex  Formal Checker

There is a formal library checker for SINGULAR which can be used online:
see @url{https://www.singular.uni-kl.de/index.php/new-libraries/formal-library-checker.html}.

After uploading your library file, you will receive an output of hints, warnings,
and errors which may help you to improve your library.

@c -------------------------------------------------------------------
@node Documentation Tool,Typesetting of help and info strings,Formal Checker,Libraries
@subsection  Documentation Tool
@cindex  Documentation Tool
@cindex lib2doc

@include lib2doc.texi

@c -------------------------------------------------------------------
@node Typesetting of help and info strings,Loading a library,Documentation Tool,Libraries
@subsection  Typesetting of help and info strings
@cindex  Typesetting of help and info strings

The info strings of the libraries which are included in the distribution
of @sc{Singular} and the help strings of the corresponding procedures
are parsed and automatically converted into the @code{texinfo} format
(the typesetting language in which the documentation  of @sc{Singular}
is written).

The illustrative example given in @ref{template_lib} should provide
sufficient information on how this works. For more details, check the
following items:

@itemize @bullet
@item
Users familiar with @code{texinfo} may write help and info strings
directly in the @code{texinfo} format. The string should, then,
start with the @code{@@} sign. In this case, no parsing will be done.
@item
Help and info strings are typeset within a @code{@@table @@asis} environment
(which is similar to the @code{latex} @code{description} environment).
@item
If a line starts with uppercase words up to a colon, then the text up to
the colon is taken to be the description-string of an item, and the
text following the colon is taken to be the content of the item.
@item
If the description-string of an item matches
@table @asis
@item @code{SEE ALSO}
then the
content of the item is assumed to consist of comma-separated words
which are valid references to other @code{texinfo}  nodes of the manual
(e.g., all procedure and command names are also @code{texinfo}  nodes).
@item  @code{KEYWORDS}
then the content of the item is assumed to be a
semicolon-separated list of phrases which are taken as keys for the
index of the manual (the name of a procedure/library is
automatically added to the index keys).
@end table
@item
If the description-string of an item in the @strong{info string of a
library} matches
@table @asis
@item @code{LIBRARY}
then the content of the item is assumed to be a one-line description of
the library. If this one-line description consists of uppercase characters
only, then it is typeset in lowercase characters (otherwise it is left as is).
@item @code{PROCEDURES}
then the content of the item is assumed to consist of lines of type
@smallexample
<proc_name>();     <one line description of the purpose>
@end smallexample
Separate @code{texinfo} nodes (subsections in printed documents)
are created precisely for those  procedures of the library appearing
here (that is, for some if not all non-static procedures of the library).
@end table
@end itemize

With respect to the content of an item, the following @code{texinfo}
markup elements are recognized:

@itemize
@item @code{@@*}
Enforces a line-break.
@table @asis
@item Example:
@code{old line @@* new line}
@* @expansion{}
@* old line @* new line
@end table

@item @code{@@ref@{...@}}
For references to other parts of the @sc{Singular} manual, use one of
the following @code{@@ref@{node@}} constructs. Here, @code{node}
must be the name of a section of the @sc{Singular} manual. In
particular,  it may be the name of a function, library or procedure
in a library.

@table @asis
@item @code{@@xref@{node@}}
for a reference to the node @code{node} at the beginning of a sentence.
@item @code{@@ref@{node@}}
for a reference to the node @code{node} at the end of a sentence.
@item @code{@@pxref@{node@}}
for a reference to the node @code{node} within parentheses.

@item Example:
@code{@@xref@{Hurricanes@}, for more info.}
@*@expansion{}*Note Hurricanes::, for more info.
@*@expansion{}See Section 3.1 [Hurricanes], page 24, for more info.

@code{For more information, see @@ref@{Hurricanes@}.}
@*@expansion{}For more information, see *Note Hurricanes::.
@*@expansion{}For more information, see Section 3.1 [Hurricanes], page 24.


@code{... storms cause flooding (@@pxref@{Hurricanes@}) ...}
@*@expansion{}... storms cause flooding (*Note Hurricanes::) ...
@*@expansion{}... storms cause flooding (see Section 3.1 [Hurricanes],
page 24)
@end table

@item @code{@@math@{..@}}
Typeset short mathematical expressions  in LaTeX math-mode
syntax (short: does not cause expansion over multiple lines).
@table @asis
@item Example:
@code{@@math@{\alpha@}}
@*@expansion{}
@*@math{\alpha}
@item Note:
The mathematical expressions inside @code{@@math@{..@}} must
not contain the characters @code{@{},@code{@}}, and @code{@@}.
@end table

@item @code{@@code@{..@}}
Typeset short strings in typewriter font (short: does not cause
expansion over multiple lines).
@table @asis
@item Example:
@code{@@code@{typewriter font@}}
@*@expansion{}
@*@code{typewriter font}
@item Note:
The string inside @code{@@code@{..@}} must not contain the
characters @code{@{},@code{@}}, and @code{@@}.
@end table

@item
Typeset pre-formatted text in typewriter font.
@example
@@example
 ...
@@end example
@end example
@table @asis
@item Example:
@example
before example
@@example
in example
notice escape of special characters like @@@{,@@@},@@@@
@@end example
after example
@end example
@expansion{}
@*before example
@example
in example
notice escape of special characters like @{,@},@@
@end example
after example
@item Note:
Inside an @@example environment, the characters @{,@},@@ have
to be escaped by an @@ sign.
@end table

@item
Typeset pre-formatted text in normal font.
@example
@@format
 ...
@@end format
@end example
@table @asis
@item Example:
@example
before format
@@format
in format
notice escape of special characters like @@@{,@@@},@@@@
@@end format
after format
@end example
@expansion{}
@*before format
@format
in format
escape of special characters like @{,@},@@
@end format
after format
@item Note:
Inside an @@format environment, the characters @{,@},@@ have
to be escaped by an @@ sign.
@end table

@item
Write text in pure @code{texinfo}.
@example
@@texinfo
 ...
@@end texinfo
@end example
@table @asis
@item Example:
@example
@@texinfo
Among others, within a texinfo environment,
one can use the tex environment to typeset
more complex mathematical items like
@@tex
$i_@{1,1@} $
@@tex
@@end texinfo
@end example
@*@expansion{}
@*Among others, within a texinfo environment, one can use the tex environment
to typeset more complex mathematical items like
@tex
$ i_{1,1} $
@end tex
@end table

Furthermore, a line-break is inserted before each line
whose previous line is shorter than 60
characters and does not contain any of the above described recognized
texinfo markup elements.
@end itemize


@c -------------------------------------------------------------------
@node Loading a library,,Typesetting of help and info strings,Libraries
@subsection   Loading a library
@cindex   Loading a library

Libraries can be loaded with the @code{LIB}  or the @code{load}  command
(@pxref{LIB} and @pxref{load}).

@c ------------------------------------------------------------
@c   This piece of text exists also in the file reference.doc,
@c   chapter "LIB".
@c   chapter "load".
@c   If you change something here, change it there, too!
@c ------------------------------------------------------------
@table @code
@item @strong{Syntax:}
@code{LIB} string_expression @code{;}
@*@code{load} string_expression @code{;}
@item @strong{Type:}
none
@item @strong{Purpose:}
Reads a library from a file. If the given filename does not start with
@kbd{.} or @kbd{/}  and if the file cannot be located in the current
directory,  the @code{SearchPath} is checked for a  directory containing
a file with this name.
@item @strong{Note on SearchPath:}
The @code{SearchPath} for a library is constructed at @sc{Singular}
start-up time as follows:
@enumerate
@item
the directories contained in the environment variable
@code{SINGULARPATH} are appended.

@item
the directories @code{$BinDir/LIB}, @code{$RootDir/LIB},
@code{$RootDir/../LIB}, @code{$DefaultDir/LIB}, @code{$DefaultDir/../LIB}
are appended, where
@itemize @bullet
@item @code{$BinDir}
is the value of the environment variable
@code{SINGULAR_BIN_DIR}, if set, or, if not set, the directory in which the
@sc{Singular} program
resides
@item @code{$RootDir}
is the value of the environment variable
@code{SINGULAR_ROOT_DIR}, if set, or, if not set, @code{$BinDir/../}.
@item @code{$DefaultDir}
is the value of the environment variable
@code{SINGULAR_DEFAULT_DIR}, if set, or @code{/usr/local}.
@end itemize

@item
all directories which do not exist are removed from the @code{SearchPath}.
@end enumerate

For setting environment variables, see @ref{system}, or consult the manual of
your shell.

The library @code{SearchPath} can be examined  by starting up
@sc{Singular} with the option @code{-v}, or by issuing the command
@code{system("--version");}.

@item @strong{Note on standard.lib:}
Unless  @sc{Singular} is started with the @code{--no-stdlib} option, the
library @code{standard.lib} is automatically loaded at start-up time.

@end table

Following a @code{LIB}  or @code{load}  command, only the names
of the procedures in the library are loaded. The body of a particular
procedure is only read upon the first call of the procedure. This
minimizes memory consumption by unused procedures. Starting
a @sc{Singular}  session with the @code{-q} or @code{--quiet}
@code{option} unsets the @code{option} @code{loadLib} and
inhibits, thus, the monitoring of library loading (see @code{option}).

All libraries loaded in a @sc{Singular}  session are displayed upon
entering @code{listvar(package);} :

@smallexample
@c example
 option(loadLib);   // show loading of libraries;
                    // standard.lib is loaded
 listvar(package);
                    // the names of the procedures of inout.lib
 LIB "inout.lib";   // are now known to Singular
 listvar(package);
@c example
@end smallexample

@c ref
See
@ref{Command line options};
@ref{Procedures and libraries};
@ref{SINGULAR libraries};
@ref{proc};
@ref{LIB};
@ref{standard_lib};
@ref{string};
@ref{system}.
@c ref


@c -------------------------------------------------------------------
@c ---------------------------------------
@c ---------------------------------------
@c ---------------------------------------
@c ---------------------------------------
@c ----------------------------------------------------------------------
@node Debugging tools, Dynamic loading, Libraries, General concepts
@section Debugging tools
@cindex Debugging tools

If @sc{Singular} does not come back to the prompt while calling a user
defined procedure, probably a bracket or a @code{"} is missing.  The
easiest way to leave the procedure is to type some brackets or @code{"}
and then @key{RETURN} .

@menu
* ASSUME::
* Tracing of procedures::
* Source code debugger::
* Break points::
* Printing of data::
* libparse::
* option(warn)::
@end menu

@c ---------------------------------------
@node ASSUME,Tracing of procedures,Debugging tools,Debugging tools
@subsection ASSUME
@cindex ASSUME

@table @code
@item @strong{Syntax:}
@code{ASSUME (} int_constant @code{,} expression @code{)}
@item @strong{Purpose:}
Tests the expression for correctness if the int_constant is smaller
as a variable @code{assumeLevel}. If no such variable exist the int expression
is compared against 0. It is possible to define an individual @code{assumeLevel} for each library and/or procedure
If the expression is evaluated and not true (i.e. does not evaluate to @code{int(0)} an error is raised.
@item @strong{Note:} @code{ASSUME} shall be used for documentation and debugging,
production code of a library must never define @code{assumeLevel}.
@item @strong{Example:}
@smallexample
@c example error
  ASSUME(0,2==2); // always tested
  ASSUME(1,1==2); // not evaluated
  int assumeLevel=2;
  ASSUME(1,1==2);
  // setting a different assumeLevel for poly.lib:
  int Poly::assumeLevel=2;
@c example
@end smallexample
@end table

@c ---------------------------------------
@node Tracing of procedures,Source code debugger,ASSUME,Debugging tools
@subsection Tracing of procedures
@cindex TRACE
Setting the @code{TRACE} variable to 1 (resp.@: 3) results in reporting of
all procedure entries and exits (resp.@: together with line numbers).
If @code{TRACE} is set to 4, @code{Singular}
displays each line before its interpretation and waits for the @key{RETURN}
key being pressed.
@menu
* TRACE var::
@end menu
@iftex
See @ref{TRACE var}.
@end iftex

@strong{Example:}
@smallexample
@c example
  proc t1
  {
    int i=2;
    while (i>0)
    { i=i-1; }
  }
  TRACE=3;
  t1();
@c example
@end smallexample

@c ---------------------------------------
@node Source code debugger, Break points, Tracing of procedures, Debugging tools
@subsection Source code debugger
@cindex debugging library code
@cindex source code debugger, sdb
@cindex debugger
@cindex sdb, source code debugger


The source code debugger (sdb) is an experimental feature, its
interface may change in future versions of @sc{Singular}.  @*To enable the
use of the source code debugger @sc{Singular} has to be started with the
option @code{-d} or @code{--sdb} (see @ref{Command line options}).


@subsubheading sdb commands
Each sdb command consists of one character which may be followed by
a parameter.
@table @asis
@item @code{b}
print backtrace of calling stack
@item @code{c}
continue
@item @code{e}
edit the current procedure and reload it (current call will be aborted)
@* only available on UNIX systems
@item @code{h},@code{?}
display help screen
@item @code{n}
execute current line, sdb break at next line
@item @code{p} <identifier>
display type and value of the variable given by <identifier>
@item @code{Q}
quit this @sc{Singular} session
@item @code{q} <flags>
quit debugger, set debugger flags(0,1,2)
@* 0: continue, disable the debugger
@* 1: continue
@* 2: throw an error, return to toplevel
@end table


@subsubheading Syntactical errors in procedures
If @sc{Singular} was started using the command line option @code{-d} or
@code{--sdb}, a syntactical error in a procedure will start the
source code debugger instead of returning to the top level with an
error message. The commands @code{q 1} and @code{q 2} are equivalent in this
case.

@subsubheading SDB breakpoints in procedures
@cindex SDB breakpoint
@cindex SDB debugger
Up to seven SDB breakpoints can be set.
To set a breakpoint at a procedure use
@code{breakpoint}. (See @ref{breakpoint}).
@*These breakpoints can be cleared with the command @code{d breakpoint_no}
from within the debugger or with
@code{breakpoint(} proc_name @code{,-1);}.


@c ---------------------------------------
@node Break points, Printing of data, Source code debugger, Debugging tools
@subsection Break points
A break point can be put into a proc by inserting the command @code{~}.
If @code{Singular} reaches a break point it asks for lines of commands
(line-length must be less than 80 characters)
from the user. It returns to normal execution if given an empty line.
See @ref{~}.


@strong{Example:}
@smallexample
proc t
@{
  int i=2;
  ~;
  return(i+1);
@}
t();
@expansion{} -- break point in t --
@expansion{} -- 0: called    from STDIN --
i;               // here local variables of the procedure can be accessed
@expansion{} 2
@expansion{} -- break point in t --

@expansion{} 3
@end smallexample
@sp 2

@c ---------------------------------------
@node Printing of data,libparse,Break points,Debugging tools
@subsection Printing of data
The procedure @code{dbprint} is useful for optional output of data:
it takes 2 arguments and prints the second argument, if the first
argument is positive; otherwise, it does nothing.
@menu
* dbprint::
* voice::
@end menu
@ifnothtml
@iftex
See @ref{dbprint}; @ref{voice}.
@end iftex
@end ifnothtml

@c ---------------------------------------
@node libparse,option(warn),Printing of data,Debugging tools
@subsection libparse
@cindex libparse

@code{libparse} is a stand-alone program contained in the @sc{Singular}
distribution (at the place where the @sc{Singular} executable program
resides), which cannot be called inside @sc{Singular}. It is a
debugging tool for libraries which performs exactly the same checks as
@ifset namespaces
the @code{load} command in @sc{Singular}, but generates more output during
@end ifset
@ifclear namespaces
the @code{LIB} command in @sc{Singular}, but generates more output during
@end ifclear
parsing.  @code{libparse} is useful if an error
occurs while loading the library, but the whole block around the line
specified seems to be correct. In these situations the real error might
have occurred hundreds of lines earlier in the library.

@strong{Usage:}
@*@code{libparse [options] singular-library}
@*@strong{Options:}
@table @asis
@item @code{-d} Debuglevel
increases the amount of output during parsing, where Debuglevel is an integer
between 0 and 4. Default is 0.
@item @code{-s}
turns on reporting about violations of unenforced syntax rules
@end table

The following syntax checks are performed in any case:
@itemize @bullet
@item counting of pairs of brackets @{,@} , [,] and (,)
    (number of @{ has to match number of @}, same for [,] and (,) ).
@item counting of "
    ( number of " must be even ).
@item general library syntax
    ( only LIB, static, proc (with parameters, help, body and example)
and comments, i.e // and @code{/* ... */}, are allowed).
@end itemize
Its output lists all procedures that have been parsed successfully:

@smallexample
$ libparse sample.lib
Checking library 'sample.lib'
  Library         function      line,start-eod line,body-eob  line,example-eoe
Version:0.0.0;
g Sample               tab line    9,  149-165   13,  271-298   14,  300-402
l Sample      internal_tab line   24,  450-475   25,  476-496    0,    0-496
@end smallexample

where the following abbreviations are used:
@itemize @bullet
@item g: global procedure (default)
@item l: static procedure, i.e., local to the library.
@end itemize

each of the following is the position of the byte in the library.
@itemize @bullet
@item start: begin of 'proc'
@item eod: end of parameters
@item body: start of procedurebody '@{'
@item eob:  end of procedurebody '@}'
@item example: position of 'example'
@item eoe: end of example '@}'
@end itemize

Hence in the above example, the first procedure of the library
sample.lib is user-accessible and its name is tab. The procedure
starts in line 9, at character 149. The head of the procedure
ends at character 165, the body starts in line 13 at character 271
and ends at character 298. The example section extends from line 14
character 300 to character 402.

The following example shows the result of a missing close-bracket @} in
line 26 of the library @code{sample.lib}.

@smallexample
LIB "sample.lib";
@expansion{}    ? Library sample.lib: ERROR occurred: in line 26, 497.
@expansion{}    ? missing close bracket '@}' at end of library in line 26.
@expansion{}    ? Cannot load library,... aborting.
@expansion{}    ? error occurred in STDIN line 1: `LIB "sample.lib";`
@end smallexample
@c ---------------------------------------
@node option(warn),,libparse,Debugging tools
@subsection option(warn)
@cindex  option(warn)
@cindex  warn, option


If this option is set some constructs which @strong{may} lead to bug
will result in a warning. While there are legitimate uses for them and they
are @strong{not errors} is is worth thinking about it.

@itemize
@item
change of options during a procedure call: is this side effect intended?
@item
use of @code{def}: avoids type checking, but useful if a procedure handles
several types at once
@item
@code{ASSUME} outside of procedures: while a failed @code{ASSUME} aborts
the current procedures and return to the top level - what should it do at top level?
@end itemize
@c ref
See
@ref{option}.
@c ref
@c ---------------------------------------
@node Dynamic loading, , Debugging tools, General concepts
@section Dynamic loading
@cindex Dynamic loading

In addition to the concept of libraries, it is also possible to
dynamically extend the functionality by loading functions written
in C/C++ or some other higher programming language. A collection
of such functions is called a dynamic module and can be loaded by
the command @code{LIB} or @code{load}. It is basically handled in
the same way as a library: upon loading, a new @code{package} is
created which holds the contents of the dynamic module.
General information about the loaded module can be displayed by the command
@code{help package_name}. After loading the dynamic module, its functions
can be used exactly like the built-in @sc{Singular} functions.

To have the full functionality of a built-in function, dynamic modules
need to comply with certain requirements on their internal structure.
As this would be beyond the scope of the @code{Singular} manual,
a separate, more detailed guide on how to write and use dynamic modules
is available.