SETREAD filename

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