Fixed the bugs introduced by the last commit! Output is now *identical* to that
produced by the old version, based on a-lists.
Gate = FOL +
consts nand,xor :: "[o,o] => o"
rules nand_def "nand(P,Q) == ~(P & Q)"
xor_def "xor(P,Q) == P & ~Q | ~P & Q"
end