intros a b r1 r2 H'; generalize r2; elim H'; clear H' a b r1 r2.
exact gcd_inv_Or.
exact gcd_inv_Ol.
intros a b c H' H'0 H'1 r2 H'2; inversion H'2; auto.
apply H'1; auto.
rewrite <- H0; simpl in |- *; rewrite H1; auto.
apply H'1; auto.
rewrite <- H; simpl in |- *; rewrite <- minus_n_O; rewrite H1; auto.
absurd (a < b); auto with arith.
intros a b c H' H'0 H'1 r2 H'2; inversion H'2; auto.
apply H'1; auto.
rewrite <- H0; simpl in |- *; rewrite H1; rewrite <- minus_n_O; auto.
apply H'1; auto.
rewrite <- H; simpl in |- *; rewrite H1; auto.
absurd (a < b); auto with arith.
Qed.

Lemma gcd_is_gcd : forall a b : nat, is_gcd a b (gcd a b).