store_thm : string * term * tactic -> thm
Proves and then stores a theorem in the current theory segment.
The call store_thm(name, t, tac) is equivalent to
save_thm(name, prove(t, tac)).
Whenever prove fails to prove the given term.
Saving theorems for retrieval in later sessions. Binding the result
of store_thm to an ML variable makes it easy to access the theorem
in the current terminal session.