PAT_X_ASSUM : term -> thm_tactic -> tactic
Finds the first assumption that matches the term argument, applies the theorem tactic to it, and removes this assumption.
The tactic
   PAT_X_ASSUM tm ttac ([A1, ..., An], g)
finds the first Ai which matches tm using higher-order pattern matching in the sense of ho_match_term. Free variables in the pattern that are also free in the assumptions or the goal must not be bound by the match. In effect, these variables are being treated as local constants.
Fails if the term doesn’t match any of the assumptions, or if the theorem-tactic fails when applied to the first assumption that does match the term.
HOL  Kananaskis-13