elim (gcd_exists_prod_bis (a,b));intro d;simpl;intros.
exists d;trivial.
Qed.

(* Specifications: *)
(** it is decidable to say if a and b are relatively prime *)
Lemma rel_prime_dec : forall (a b:nat),{rel_prime a b}+{~(rel_prime a b)}.
intros.