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.;
NatSum = Main +
consts sum :: nat => nat
primrec
"sum 0 = 0"
"sum (Suc n) = Suc n + sum n"
end