OPENUPDATE filename
command. Opens the named file for reading and writing. The read and
write position is initially set to the end of the old file, if any.
Note: each open file has only one position, for both reading and
writing. If a file opened for update is both READER and WRITER at
the same time, then SETREADPOS will also affect WRITEPOS and vice
versa. Also, if you alternate reading and writing the same file,
you must SETREADPOS between a write and a read, and SETWRITEPOS
between a read and a write.