`HINT_EXISTS_TAC : tactic`

SYNOPSIS
Attemps to instantiate existential goals from context.

DESCRIPTION
Given a goal which contains some subformula of the form ?x_1... x_k. P_1 y^1_1 ... y^1_m1 /\ ... /\ P_n y^n_1 ... y^n_mn in a context where P_i t_1 ... t_mi holds for some t_1,...,t_mi, then instantiates x_i1,...,x_i_mi with t_1,...,t_mi. The ``context'' consists in the assumptions or in the premisses of the implications where the existential subformula occurs. Note: it is enough that just P t holds, not the complete existentially quantified formula. As the name suggests, we just use the context as a ``hint'' but it is (in most general uses) not sufficient to solve the existential completely: if this is doable automatically, then other techniques can do the job in a better way (typically MESON).

FAILURE CONDITIONS
Fails if no instantiation is found from the context.

EXAMPLE
```  # g `!P Q R S. P 1 /\ Q 2 /\ R 3 ==> ?x y. P x /\ R y /\ S x y`;;
val it : goalstack = 1 subgoal (1 total)

`!P Q R S. P 1 /\ Q 2 /\ R 3 ==> (?x y. P x /\ R y /\ S x y)`

# e HINT_EXISTS_TAC;;
val it : goalstack = 1 subgoal (1 total)

`!P Q R S. P 1 /\ Q 2 /\ R 3 ==> S 1 3`
```

USES
When facing an existential goal, it happens often that the context ``suggests'' a candidate to be a witness. In many cases, this is because the existential goal is partly satisfied by a proposition in the context. However, often, the context does not allow to automatically prove completely the existential using this witness. Therefore, usual automation tactics are useless. Usually, in such circumstances, one has to provide the witness explicitly. This is tedious and time-consuming whereas this witness can be found automatically from the context, this is what this tactic allows to do.