cut (forall (m : nat) a, sizel a < m -> Acc lessP a).
intros H' a.
apply H' with (m := S (sizel a)); auto.
intros m; elim m; clear m.
intros a H; inversion H.
intros m H' a H'0.
apply Acc_intro.
intros y H'1; apply H'.
unfold lessP in H'1.
apply lt_le_trans with (sizel a); auto with arith.
Qed.
Hint Resolve wf_lessP.

Theorem pX_inj :
forall (n1 n2 : Term A n) l1 l2, n1 = n2 -> l1 = l2 -> pX n1 l1 = pX n2 l2.