intros x y z H' H'0; case (Zle_lt_or_eq _ _ H'); intros Zlt1.
apply Zmult_gt_0_le_compat_r; auto.
apply Zlt_gt; auto.
rewrite <- Zlt1; repeat rewrite <- Zmult_0_r_reverse; auto with zarith.
Qed.

(* Specifications: *)
Theorem Zle_Zmult_comp_l :
forall x y z : Z, (0 <= z)%Z -> (x <= y)%Z -> (z * x <= z * y)%Z.