added incremental extension functions: extend_log_types, extend_type_gram,
extend_const_gram, extend_consts, extend_trfuns, extend_trrules;
replaced merge by merge_syntaxes;
various minor internal changes;
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