`PART_MATCH : (term -> term) -> thm -> term -> thm`

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. Limited higher-order matching is supported, and some attempt is made to maintain bound variable names in higher-order matching.

FAILURE CONDITIONS
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. For a higher-order case rather similar to what goes on inside HOL's INDUCT_TAC:
```  # num_INDUCTION;;
val it : thm = |- !P. P 0 /\ (!n. P n ==> P (SUC n)) ==> (!n. P n)

# PART_MATCH rand it `!n. n <= n * n`;;
val it : thm =
|- 0 <= 0 * 0 /\ (!n. n <= n * n ==> SUC n <= SUC n * SUC n)
==> (!n. n <= n * n)
```
To show a more interesting case with higher-order matching, where the pattern is not quite a higher-order pattern in the usual sense, consider the theorem:
```  # let th = MESON[num_CASES; NOT_SUC]
`(!n. P(SUC n)) <=> !n. ~(n = 0) ==> P n`
...
val th : thm = |- (!n. P (SUC n)) <=> (!n. ~(n = 0) ==> P n)
```
and instantiate it as follows:
```  # PART_MATCH lhs th `!n. 1 <= SUC n`;;
val it : thm = |- (!n. 1 <= SUC n) <=> (!n. ~(n = 0) ==> 1 <= n)
```