elim (bezout_exists_prod (a,b));intro.
elim a0;destruct x;simpl;intros.
left;exists n;exists n0;rewrite mult_comm;rewrite (mult_comm b);trivial.
elim b0;destruct x;simpl;intros.
right;exists n;exists n0;rewrite mult_comm;rewrite (mult_comm a);trivial.
Qed.

(* Specifications: *)
(** Bezout' theorem reformulated *)
Theorem bezout : forall (d a b:nat),(is_gcd d a b)->exists u:nat,exists v:nat,d=a*u-b*v \/ d=b*v-a*u.
intros.