intros a b p q H'; rewrite (plusP_inv2 a b p q (pluspf (pX a p) (pX b q)));
auto.
Qed.

Theorem pluspf_inv3a_eq :
forall a b p q,
eqT a b ->
zeroP (A:=A) A0 eqA (n:=n) (plusTerm (A:=A) plusA (n:=n) a b) ->
pluspf p q = pluspf (pX a p) (pX b q).