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.