intros.
split.
red;split;[apply divides_refl | apply one_min_div].
unfold is_cd;tauto.
Qed.

(** if a <= b then gcd(a,b)=gcd(a,b-a) *)
Lemma gcd_minus : forall (d a b:nat),(a <= b)->((is_gcd d a b)<->(is_gcd d a (b-a))).
intros.