When applied to a goal A ?- !x::P. t, the tactic RESQ_GEN_TAC
reduces it to a new goal A,P x' ?- t[x'/x] where x' is a variant of x
chosen to avoid clashing with any variables free in the goal’s
assumption list. Normally x' is just x.
A ?- !x::P. t
=================== RESQ_GEN_TAC
A,P x' ?- t[x'/x]