FILTER_GEN_TAC : (term -> tactic)
Strips off a universal quantifier, but fails for a given quantified variable.
When applied to a term s and a goal A ?- !x. t, the tactic FILTER_GEN_TAC fails if the quantified variable x is the same as s, but otherwise advances the goal in the same way as GEN_TAC, i.e. returns the goal 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
   ==============  FILTER_GEN_TAC "s"
    A ?- t[x'/x]

Fails if the goal’s conclusion is not universally quantified or the quantified variable is equal to the given term.
HOL  Kananaskis-14