intros n1 n2 l1 l2 H' H'0; rewrite H'0; rewrite H'; auto.
Qed.
Hint Resolve pX_inj.

(* Specifications: *)
Lemma pX_invl : forall (a b : Term A n) p q, pX a p = pX b q -> a = b.
intros a b p q H'; inversion H'; auto.