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

(* Specifications: *)
Theorem pluspf_inv3a :
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 (pluspf p q) (pluspf (pX a p) (pX b q)).