proposition: Lemma gcd_mul_mono_l_nonneg : forall n m p, 0<=p -> gcd (p*n) (p*m) == p * gcd n m.
proof: Proof. intros n m p ?. rewrite <- (abs_eq p) at 3; trivial. apply gcd_mul_mono_l. Qed.
imports: ZAxioms ZMulOrder ZSgnAbs NZGcd