intros a b p q H' Z; try assumption.
rewrite (plusP_inv3a a b p q (pluspf (pX a p) (pX b q))); auto.
Qed.

Theorem pluspf_inv3b :
forall a b p q,
eqT a b ->
~ zeroP (A:=A) A0 eqA (n:=n) (plusTerm (A:=A) plusA (n:=n) a b) ->
eqP A eqA n (pX (plusTerm (A:=A) plusA (n:=n) a b) (pluspf p q))
(pluspf (pX a p) (pX b q)).