SETPREFIX string

	command.  Sets a prefix that will be used as the implicit beginning
	of filenames in OPENREAD, OPENWRITE, OPENAPPEND, OPENUPDATE, LOAD,
	and SAVE commands.  Logo will put the appropriate separator
	character (slash for Unix, backslash for DOS/Windows, colon for
	MacOS) between the prefix and the filename entered by the user.
	The input to SETPREFIX must be a word, unless it is the empty list,
	to indicate that there should be no prefix.