intros a b p q H' Z; try assumption.
rewrite (plusP_inv3b a b p q (pluspf (pX a p) (pX b q))); auto.
Qed.
Hint Resolve pluspf_inv1 pluspf_inv2 pluspf_inv3a pluspf_inv3b.

Theorem plusP_com :
forall p q r s, plusP p q r -> plusP q p s -> eqP A eqA n r s.