cleanup of former settings GHC_PATH, EXEC_GHC, EXEC_OCAML, EXEC_SWIPL, EXEC_YAP -- discontinued implicit detection;
determine swipl_version at runtime;
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