intros l r avl_l avl_r.
intro b; elim b; clear b; intro bal0.
apply Is_Left_Balanced.
rewrite <- (avl_height_avl_height r avl_r).
rewrite <- (avl_height_avl_height l avl_l).
apply inv_left_balanced_avl; assumption.
apply Is_Fully_Balanced.
rewrite <- (avl_height_avl_height r avl_r).
rewrite <- (avl_height_avl_height l avl_l).
apply inv_fully_balanced_avl; assumption.
apply Is_Right_Balanced.
rewrite <- (avl_height_avl_height r avl_r).
rewrite <- (avl_height_avl_height l avl_l).
apply inv_right_balanced_avl; assumption.
Qed.

(* Specifications: *)
Lemma is_avl_is_balanced :
forall (l r : avl_tree) (k : Int) (d : B) (b : bal),
is_avl (Avl_Node k d l r b) -> is_balanced l r b.