proposition: Lemma divide_div : forall a b c, a~=0 -> (a|b) -> (b|c) -> (b/a|c/a).
proof: Proof. intros a b c Ha Hb (c',Hc). exists c'. now rewrite <- divide_div_mul_exact, <- Hc. Qed.
imports: ZAxioms ZMulOrder ZSgnAbs ZGcd ZDivTrunc ZDivFloor