intros a; exists a; auto.
intros b; exists b; auto.
intros a b Rr; case Rr; intros r Hr; exists r.
apply gcd_spec_ex3; auto with arith.
rewrite plus_comm; rewrite minus_plus; auto; rewrite <- minus_n_n; auto.
intros a b Rr; case Rr; intros r Hr; exists r.
case (zerop a); intros lta.
rewrite lta; simpl in |- *.
rewrite <- (gcd_inv_Or b r); auto.
apply gcd_spec_ex3; auto with arith.
rewrite <- minus_n_n; auto.
rewrite <- lta; auto.
apply gcd_spec_ex2; auto with arith.
pattern b at 1 in |- *; replace b with (0 + b); auto with arith.
rewrite plus_comm; rewrite minus_plus; auto; rewrite <- minus_n_n; auto.
Qed.
Definition gcd (a b : nat) := proj1_sig (gcd_ex a b).

Lemma gcd_correct : forall a b : nat, gcd_spec a b (gcd a b).
Hint Resolve gcd_correct.