Strips the outermost universal quantifier from the conclusion of a goal.
When applied to a goal A ?- !x. t, the tactic GEN_TAC reduces it to
A ?- t[x'/x] where x' is a variant of x chosen to avoid clashing with any
variables free in the goal's assumption list. Normally x' is just x.
A ?- !x. t
A ?- t[x'/x]
Fails unless the goal's conclusion is universally quantified.
The tactic REPEAT GEN_TAC strips away any universal quantifiers, and
is commonly used before tactics relying on the underlying term structure.