intros a b p q H' Z; try assumption.
rewrite (minusP_inv3b a b p q (minuspf (pX a p) (pX b q))); auto.
Qed.

(* Specifications: *)
Theorem minuspf_inv1 :
forall (a b : Term A n) (p q : list (Term A n)),
ltT ltM b a ->
eqP A eqA n (pX a (minuspf p (pX b q))) (minuspf (pX a p) (pX b q)).