When applied to a term x', which should be a variable, and a goal
A ?- !x. t, the tactic X_GEN_TAC returns the goal A ?- t[x'/x].
A ?- !x. t
============== X_GEN_TAC "x'"
A ?- t[x'/x]
FAILURE
Fails unless the goal’s conclusion is universally quantified and the term a
variable of the appropriate type. It also fails if the variable given is free
in either the assumptions or (initial) conclusion of the goal.