`SUBST_MATCH : (thm -> thm -> thm)`
STRUCTURE
SYNOPSIS
Substitutes in one theorem using another, equational, theorem.
DESCRIPTION
Given the theorems A|-u=v and A'|-t, SUBST_MATCH (A|-u=v) (A'|-t) searches for one free instance of u in t, according to a top-down left-to-right search strategy, and then substitutes the corresponding instance of v.
```    A |- u=v   A' |- t
--------------------  SUBST_MATCH (A|-u=v) (A'|-t)
A u A' |- t[v/u]
```
SUBST_MATCH allows only a free instance of u to be substituted for in t. An instance which contain bound variables can be substituted for by using rewriting rules such as REWRITE_RULE, PURE_REWRITE_RULE and ONCE_REWRITE_RULE.
FAILURE
SUBST_MATCH th1 th2 fails if the conclusion of the theorem th1 is not an equation. Moreover, SUBST_MATCH (A|-u=v) (A'|-t) fails if no instance of u occurs in t, since the matching algorithm fails. No change is made to the theorem (A'|-t) if instances of u occur in t, but they are not free (see SUBS).
EXAMPLE
```   - val thm1 = SPECL [Term `m:num`, Term `n:num`] arithmeticTheory.ADD_SYM;
> val thm1 = |- m + n = n + m : thm
```
is used to apply substitutions, depending on the occurrence of free instances
```   - SUBST_MATCH thm1 (ASSUME (Term `(n + 1) + (m - 1) = m + n`));
> val it =  [.] |- m - 1 + (n + 1) = m + n : thm

- SUBST_MATCH thm1 (ASSUME (Term `!n. (n + 1) + (m - 1) = m + n`));
> val it =  [.] |- !n. n + 1 + (m - 1) = m + n : thm
```

USES
SUBST_MATCH is used when rewriting with the rules such as REWRITE_RULE, using a single theorem is too extensive or would diverge. Moreover, applying SUBST_MATCH can be much faster than using the rewriting rules.
SEEALSO