intros n m p H1 H2; replace (p * n)%Z with (-(-p * n))%Z; auto with zarith; try ring.
replace (p * m)%Z with (-(-p * m))%Z; auto with zarith; try ring.
apply Zopp_gt; apply Zmult_gt_compat_l; auto with zarith.
Qed.
(* Theorem to simplify a hyp x * y ? x * z where ? is < > <= >= *)

Theorem Zmult_le_compat_l_rev:
forall n m p : Z, (0 < p)%Z -> (p * n <= p * m)%Z -> (n <= m)%Z.