I'm thinking that instead of #f a singleton should be used instead that has no other purpose.  



John Cowan          http://vrici.lojban.org/~cowan        xxxxxx@ccil.org
We do, doodley do, doodley do, doodley do,
What we must, muddily must, muddily must, muddily must;
Muddily do, muddily do, muddily do, muddily do,
Until we bust, bodily bust, bodily bust, bodily bust.  --Bokonon