Equivalence under nfix, i.e., natural number equivalence.
Theorem:
(defthm int-equiv-refines-nat-equiv (implies (int-equiv x y) (nat-equiv x y)) :rule-classes (:refinement))
Theorem:
(defthm nat-equiv-implies-equal-zp-1 (implies (nat-equiv a a-equiv) (equal (zp a) (zp a-equiv))) :rule-classes (:congruence))