intros n m H'; case (Zle_lt_or_eq _ _ H'); auto with zarith.
Qed.

(* Specifications: *)
Theorem Zlt_Zopp_Inv : forall p q : Z, (- p < - q)%Z -> (q < p)%Z.
intros x y H'; case (Zle_or_lt x y); auto with zarith.