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

Lemma Zeven_def2 : forall z : Z, (exists k : Z, z = 2 * k) -> (Zeven z).