intros a b p q H'; try assumption.
rewrite (minusP_inv1 a b p q (minuspf (pX a p) (pX b q))); auto.
Qed.

Theorem minuspf_inv2_eq :
forall (a b : Term A n) (p q : list (Term A n)),
ltT ltM a b ->
pX (invTerm (A:=A) invA (n:=n) b) (minuspf (pX a p) q) =
minuspf (pX a p) (pX b q).