PMATCH_MP : (thm -> thm -> thm)
Modus Ponens inference rule with automatic matching.
When applied to theorems A1 |- ! t1 ==> t2 and A2 |- t1', the inference rule PMATCH_MP matches t1 to t1' by instantiating free or paired universally quantified variables in the first theorem (only), and returns a theorem A1 u A2 |- ! t2', where t2' is a correspondingly instantiated version of t2. Polymorphic types are also instantiated if necessary.

Variables free in the consequent but not the antecedent of the first argument theorem will be replaced by variants if this is necessary to maintain the full generality of the theorem, and any pairs which were universally quantified over in the first argument theorem will be universally quantified over in the result, and in the same order.

    A1 |- ! t1 ==> t2   A2 |- t1'
   --------------------------------------  MATCH_MP
          A1 u A2 |- ! t2'

Fails unless the first theorem is a (possibly repeatedly paired universally quantified) implication whose antecedent can be instantiated to match the conclusion of the second theorem, without instantiating any variables which are free in A1, the first theorem’s assumption list.
HOL  Kananaskis-14