proposition: Lemma divide_opp_r : forall n m, (n | -m) <-> (n | m).
proof: Proof. intros n m. split; intros (p,Hp); exists (-p). - now rewrite mul_opp_l, <- Hp, opp_involutive. - now rewrite Hp, mul_opp_l. Qed.
imports: ZAxioms ZMulOrder ZSgnAbs NZGcd