Proof using.
intros n n1 ln t l H; generalize n1; elim H; clear H n ln t l n1;
auto with arith.
intros n t n1 [H2| H2]; [ rewrite H2 | case H2 ]; auto.
intros n ln1 ln2 t1 t2 l1 l2 H H0 H1 H2 n1 H3; apply le_trans with (S n);
auto with arith.
case in_app_or with (1 := H3); auto.
Qed.

(* Specifications: *)
(* In the height list is not a singleton, all its element are strictly larger than the initial height *)
Theorem height_pred_larger_strict :
forall (n n1 : nat) (ln : list nat) (t : btree A) (l : list (btree A)),
height_pred n ln l t -> In n1 ln -> n < n1 \/ ln = n :: nil /\ l = t :: nil.