Reduces existentially quantified goal to one involving a specific witness.
DESCRIPTION
When applied to a term u and a goal ?x. t, the tactic
EXISTS_TAC reduces the goal to t[u/x] (substituting u
for all free instances of x in t, with variable renaming if
necessary to avoid free variable capture).
A ?- ?x. t
============= EXISTS_TAC "u"
A ?- t[u/x]
FAILURE
Fails unless the goal’s conclusion is existentially quantified and the
term supplied has the same type as the quantified variable in the goal.