FILTER_STRIP_THEN : (thm_tactic -> term -> tactic)
Conditionally strips a goal, handing an antecedent to the theorem-tactic.
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.

FILTER_STRIP_THEN ttac u (A,t) fails if t is not a universally quantified term, an implication, a negation or a conjunction; or if the term being stripped contains the term u (conjunction excluded); or if the application of ttac fails, after stripping the goal.
When solving the goal
   ?- (n = 1) ==> (n * n = n)
the application of FILTER_STRIP_THEN SUBST1_TAC "m:num" results in the goal
   ?- 1 * 1 = 1

FILTER_STRIP_THEN is used when manipulating intermediate results using theorem-tactics, after stripping outer connectives from a goal in a more delicate way than STRIP_GOAL_THEN.
HOL  Kananaskis-14