I'd suggest targeting abs(x-.5), or max(x,.5). Then you could go after piecewise linear, which would solve your puzzle. OTOH, if you can't do abs or max, you've got a negative answer.

--The ASU approximation puzzle has been solved (by me) I think... the proof still could use a little fleshing out, but I think it is valid and it also is interesting... you can read it in section 8 of this pre-paper http://dl.dropbox.com/u/3507527/SpeechProc.html which also explains where this puzzle came from and what it is useful for (ASU stands for Aho Steiglitz Ullman).

You don't mention the 'closed under composition' property. Rich

--good point, will do.