intros.
elim H;intro q;intro.
exists (b * q).
rewrite H0.
ring.
Qed.

(* Specifications: *)
(** if d | a and d | b then d | (b-a) *)
Lemma divides_minus : forall (d a b:nat),(divides a d)->(divides b d)->(divides (b-a) d).
unfold divides.