TAC_PROOF : goal * tactic -> thm
Attempts to prove a goal using a given tactic.
When applied to a goal-tactic pair (A ?- t,tac), the TAC_PROOF function
attempts to prove the goal A ?- t, using the tactic tac. If it succeeds, it
returns the theorem A' |- t corresponding to the goal, where the assumption
list A' may be a proper superset of A unless the tactic is valid; there
is no inbuilt validity checking.
Fails unless the goal has hypotheses and conclusions all of type bool,
and the tactic can solve the goal.