_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.
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.)