#[global]
Instance gcd_wd : Proper (eq==>eq==>eq) gcd.

(* Specifications: *)
Lemma gcd_divide_iff : forall n m p, (p | gcd n m) <-> (p | n) /\ (p | m).
Lemma gcd_unique_alt : forall n m p, 0<=p -> (forall q, (q|p) <-> (q|n) /\ (q|m)) -> gcd n m == p.
Lemma gcd_comm : forall n m, gcd n m == gcd m n.
Lemma gcd_assoc : forall n m p, gcd n (gcd m p) == gcd (gcd n m) p.
Lemma gcd_0_l_nonneg : forall n, 0<=n -> gcd 0 n == n.
Lemma gcd_0_r_nonneg : forall n, 0<=n -> gcd n 0 == n.
Lemma gcd_1_l : forall n, gcd 1 n == 1.
Lemma gcd_1_r : forall n, gcd n 1 == 1.
Lemma gcd_diag_nonneg : forall n, 0<=n -> gcd n n == n.
Lemma gcd_eq_0_l : forall n m, gcd n m == 0 -> n == 0.
Lemma gcd_eq_0_r : forall n m, gcd n m == 0 -> m == 0.
Lemma gcd_eq_0 : forall n m, gcd n m == 0 <-> n == 0 /\ m == 0.
Lemma gcd_mul_diag_l : forall n m, 0<=n -> gcd n (n*m) == n.
Lemma divide_gcd_iff : forall n m, 0<=n -> ((n|m) <-> gcd n m == n).