Given a theorem-tactic ttac, a term u and a goal (A,t),
FILTER_STRIP_THEN ttac u removes one outer connective (!, ==>, or ~)
from t, if the term being stripped does not contain a free instance of u. A
negation ~t is treated as the implication t ==> F. The theorem-tactic
ttac is applied only when stripping an implication, by using the antecedent
stripped off. FILTER_STRIP_THEN also breaks conjunctions.
FILTER_STRIP_THEN behaves like STRIP_GOAL_THEN, if the term being stripped
does not contain a free instance of u. In particular,
FILTER_STRIP_THEN STRIP_ASSUME_TAC behaves like FILTER_STRIP_TAC.