Definition distinct_parity (a b : Z) :=
(Zeven a) /\ (Zodd b) \/ (Zodd a) /\ (Zeven b).

Lemma not_rel_prime1 : forall x y : Z,
~ rel_prime x y -> exists d : Z, Zis_gcd x y d /\ d <> 1 /\ d <> -1.