`UNIFY_ACCEPT_TAC : term list -> thm -> 'a * term -> ('b list * instantiation) * 'c list * (instantiation -> 'd list -> thm)`

SYNOPSIS
Unify free variables in theorem and metavariables in goal to accept theorem.

DESCRIPTION
Given a list l of assignable metavariables, a theorem th of the form A |- t and a goal A' ?- t', the tactic UNIFY_ACCEPT_TAC attempts to unify t and t' by instantiating free variables in t and metavariables in the list l in the goal t' so that they match, then accepts the theorem as the solution of the goal.

FAILURE CONDITIONS
Fails if no unification will work. In fact, type instantiation is not at present included in the unification.

EXAMPLE
An inherently uninteresting but instructive example is the goal:
```  # g `(?x:num. p(x) /\ q(x) /\ r(x)) ==> ?y. p(y) /\ (q(y) <=> r(y))`;;
```
which could of course be solved directly by MESON_TAC[] or ITAUT_TAC. In fact, the process we will outline is close to what ITAUT_TAC does automatically. Let's start with:
```  # e STRIP_TAC;;
val it : goalstack = 1 subgoal (1 total)

0 [`p x`]
1 [`q x`]
2 [`r x`]

`?y. p y /\ (q y <=> r y)`
```
and defer the actual choice of existential witness by introducing a metavariable:
```  # e (X_META_EXISTS_TAC `n:num` THEN CONJ_TAC);;
val it : goalstack = 2 subgoals (2 total)

0 [`p x`]
1 [`q x`]
2 [`r x`]

`q n <=> r n`

0 [`p x`]
1 [`q x`]
2 [`r x`]

`p n`
```
Now we finally fix the metavariable to match our assumption:
```  # e(FIRST_X_ASSUM(UNIFY_ACCEPT_TAC [`n:num`]));;
val it : goalstack = 1 subgoal (1 total)

0 [`p x`]
1 [`q x`]
2 [`r x`]

`q x <=> r x`
```
Note that the metavariable has also been correspondingly instantiated in the remaining goal, which we can solve easily:
```  # e(ASM_REWRITE_TAC[]);;
val it : goalstack = No subgoals
```

USES
Terminating proof search when using metavariables. Used in ITAUT_TAC