no pervasive bindings;
removed theory_results and related hook;
print_results: ignore empty/internal kind -- like former theory_results;
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