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.;
ConstDefs = Types +
constdefs nand :: gate
"nand A B == ~(A & B)"
exor :: gate
"exor A B == A & ~B | ~A & B"
end