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 terminal, as usual.  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;
	when another SETWRITE is done, the contents of the buffer (as an
	unparsed word, which may contain newline characters) will become
	the value of the named variable.  Changing the write stream does
	not close the file that was previously the write stream, so it is
	possible to alternate between files.

