Qed.
(* Theorems to simplify the hyp 0 ? x * y and x * y ? 0 where ? is < > <= >= *)

Theorem Zle_sign_pos_pos_rev: forall x y: Z, (0 < x -> 0 <= x * y -> 0 <= y)%Z.
intros x y H1 H2; case (Zle_or_lt 0 y); auto with zarith.