intros.
elim H;intros.
split.
red;red in H0;tauto.
intros.
apply H1.
red;red in H2;tauto.
Qed.

(* Specifications: *)
(** gcd(0,a)=a *)
Lemma gcd_zero : forall (a:nat),(is_gcd a O a).
unfold is_gcd.