-- Oops, no, it's not a Toffoli gate unless it has three outputs, and my "smaller" solution absorbs the first two signals, so we'd need signal splitters on those inputs, or better, on the inputs to calcyman's smaller C XOR (A AND B) mechanism:

http://conwaylife.com/forums/viewtopic.php?f=&p=57193#p57196

I'd better find something else to post about to keep this thread open, where I won't make such silly mistakes. Might have to go back and make some progress on the new metacell.