intros a b p q H' Z; try assumption.
rewrite (minusP_inv3a a b p q (minuspf (pX a p) (pX b q))); auto.
Qed.

Theorem minuspf_inv3b :
forall (a b : Term A n) (p q : list (Term A n)),
eqT a b ->
~ zeroP (A:=A) A0 eqA (n:=n) (minusTerm (A:=A) minusA (n:=n) a b) ->
eqP A eqA n (pX (minusTerm (A:=A) minusA (n:=n) a b) (minuspf p q))
(minuspf (pX a p) (pX b q)).