corrected problem with auto_tac: now uses a variant of depth_tac that avoids
interference of the simplifier with dup_step_tac
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