.MAYBEOUTPUT value					(special form)

	works like OUTPUT except that the expression that provides the
	input value might not, in fact, output a value, in which case
	the effect is like STOP.  This is intended for use in control
	structure definitions, for cases in which you don't know whether
	or not some expression produces a value.  Example:

		to invoke :function [:inputs] 2
		.maybeoutput apply :function :inputs
		end

		? (invoke "print "a "b "c)
		a b c
		? print (invoke "word "a "b "c)
		abc

	This is an alternative to RUNRESULT.  It's fast and easy to use,
	at the cost of being an exception to Logo's evaluation rules.
	(Ordinarily, it should be an error if the expression that's
	supposed to provide an input to something doesn't have a value.)