SETWRITE filename

	command.  Makes the named file the write stream, used for PRINT,
	etc.  The file must already be open with OPENWRITE, OPENAPPEND, or
	OPENUPDATE.  If the input is the empty list, then the write stream
	becomes the screen, as usual.  Changing the write stream does
	not close the file that was previously the write stream, so it is
	possible to alternate between files.

	If the input is a list, then its first element must be a variable
	name, and its second and last element must be a positive integer; a
	buffer of that many characters will be allocated, and will become the
	writestream.  If the same list (same in the .EQ sense, not a copy)
	has been used as input to OPENWRITE, then the already-allocated
	buffer will be used, and the writer can be changed to and from this
	buffer, with all the characters accumulated as in a file.  When the
	same list is used as input to CLOSE, the contents of the buffer
	(as an unparsed word, which may contain newline characters) will
	become the value of the named variable.  For compatibility with
	earlier versions, if the list has not been opened when the SETWRITE
	is done, it will be opened implicitly, but the first SETWRITE after
	this one will implicitly close it, setting the variable and freeing
	the allocated buffer.