intros p; elim p.
intros q H'; inversion H'; auto.
intros a l H' q H'0; inversion H'0; auto.
Qed.

(* Specifications: *)
Theorem pO_minusP_inv2 :
forall p q : list (Term A n), minusP p (pO A n) q -> p = q.
Theorem minusP_inv1 :
forall (a b : Term A n) (p q s : list (Term A n)),
minusP (pX a p) (pX b q) s -> ltT ltM b a -> s = pX a (minuspf p (pX b q)).