Specializes a goal with the given paired structure of variables.
When applied to a paired structure of variables p', and a goal
A ?- !p. t, the tactic P_PGEN_TAC returns the goal A ?- t[p'/p].
A ?- !p. t
============== P_PGEN_TAC "p'"
A ?- t[p'/x]
Fails unless the goal’s conclusion is a paired universal quantification
and the term a paired structure of variables of the appropriate type.
It also fails if any of the variables of the supplied structure occurs free
in either the assumptions or (initial) conclusion of the goal.