PART_MATCH : (term -> term) -> thm -> term -> thm
STRUCTURE
Drule
SYNOPSIS
Instantiates a theorem by matching part of it to a term.
DESCRIPTION
When applied to a ‘selector’ function of type term -> term, a theorem and a term: 
   PART_MATCH fn (A |- !x1...xn. t) tm
the function PART_MATCH applies fn to t' (the result of specializing universally quantified variables in the conclusion of the theorem), and attempts to match the resulting term to the argument term tm. If it succeeds, the appropriately instantiated version of the theorem is returned.
FAILURE
Fails if the selector function fn fails when applied to the instantiated theorem, or if the match fails with the term it has provided.
EXAMPLE
Suppose that we have the following theorem: 
   th = |- !x. x==>x
then the following: 
   PART_MATCH (fst o dest_imp) th "T"
results in the theorem: 
   |- T ==> T
because the selector function picks the antecedent of the implication (the inbuilt specialization gets rid of the universal quantifier), and matches it to T.
SEEALSO
INST_TYPE, INST_TY_TERM, match_term
HOL  Kananaskis-10