(* Specifications: *)
Lemma gcd_unicity_apart_sign : forall a b c d : Z, is_gcd Z IdZ Zmult 0%Z a b c -> is_gcd Z IdZ Zmult 0%Z a b d -> d = c \/ d = (- c)%Z.
Lemma gcd_OZ_absZ : forall b : Z, is_gcd Z IdZ Zmult 0%Z 0%Z b (Zabs b).