If the goal is
?- !x. x * 2 < y * (z + 1) * (y + a)
then applying Q.MATCH_GOALSUB_RENAME_TAC `y + c` will match the
pattern y + c against the various subterms within the goal. The
first obvious match, with z + 1 will be rejected because the
variable y is free in the goal, and is treated as if it were a local
constant. Because of this, y + a is the matching sub-term, and after
renaming the goal becomes
?- !x. x * 2 < y * (z + 1) * (y + c)