Q.REFINE_EXISTS_TAC : term quotation -> tactic
Attacks existential goals, making the existential variable more concrete.
The tactic Q.REFINE_EXISTS_TAC q parses the quotation q in the context of the (necessarily existential) goal to which it is applied, and uses the resulting term as the witness for the goal. However, if the witness has any variables not already present in the goal, then these are treated as new existentially quantified variables. If there are no such “free” variables, then the behaviour is the same as EXISTS_TAC.
Fails if the goal is not existential, or if the quotation can not parse to a term of the same type as the existentially quantified variable.
If the quotation doesn’t mention any new variables:
   - Q.REFINE_EXISTS_TAC `n` ([``n > x``], ``?m. m > x``);
   > val it =
       ([([``n > x``], ``n > x``)], fn)
       : (term list * term) list * (thm list -> thm)
If the quotation does mention any new variables, they are existentially quantified in the new goal:
   - Q.REFINE_EXISTS_TAC `n + 2` ([``~P 0``], ``?p. P (p - 1)``);
   > val it =
       ([([``~P 0``], ``?n. P (n + 2 - 1)``)], fn)
       : (term list * term) list * (thm list -> thm)

Q.REFINE_EXISTS_TAC is useful if it is clear that an existential goal will be solved by a term of particular form, while it is not yet clear precisely what term this will be. Further proof activity should be able to exploit the additional structure that has appeared in the place of the existential variable.
HOL  Kananaskis-14