Proof using.
intros n ln t l H; elim H; simpl in |- *; auto.
intros; discriminate.
intros n0 ln1 ln2 t1 t2 l1 l2 H0; case l1; simpl in |- *; auto.
intros; discriminate.
Qed.

(* Specifications: *)
(* The height and cover lists have same length *)
Theorem height_pred_length :
forall (n : nat) (ln : list nat) (t : btree A) (l : list (btree A)),
height_pred n ln l t -> length ln = length l.