removed cases_of;
renamed cases_tac to case_tac; tuned to work with basic HOL as well;
add_cases_induct: proper case names;
adapted to new PureThy.add_thms etc.;
Defs = Types +
consts nand, exor :: gate
defs nand_def "nand A B == ~(A & B)"
exor_def "exor A B == A & ~B | ~A & B"
end