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 is_sqr_compat : forall k a : Z, k <> 0 -> is_sqr ((k * k) * a) -> is_sqr a.