intros a b p q H'; try assumption.
rewrite (plusP_inv1 a b p q (pluspf (pX a p) (pX b q))); auto.
Qed.

(* Specifications: *)
Theorem pluspf_inv2 :
forall a b p q,
ltT ltM a b ->
eqP A eqA n (pX b (pluspf (pX a p) q)) (pluspf (pX a p) (pX b q)).