proposition: Lemma Zgcd_pos : forall a b : Z, a <> 0%Z \/ b <> 0%Z -> (0 < Zgcd a b)%Z.
proof: Proof. intros a b Hab. generalize (Zgcd_nonneg a b); intro Hnonneg. cut (Zgcd a b <> 0%Z). auto with zarith. intro H0. generalize (Zgcd_zero a b H0). tauto. Qed.
imports: CoRN.model.Zmod.ZDivides Coq.Init.Wf Coq(Lia)