Proof using.
intros n ln t l H; elim H; simpl in |- *; auto.
Qed.

(* Specifications: *)
(* The height list is never empty *)
Theorem height_pred_not_nil1 :
forall (n : nat) (ln : list nat) (t : btree A) (l : list (btree A)),
height_pred n ln l t -> ln <> nil.