intros key k d l r b data avl_t equal lookup_t.
elim (inv_lookup key k d l r b data lookup_t); intro u0.
elim u0; trivial.
elimtype False.
elim u0; clear u0; intro u0.
apply (lookup_below_false key l data u0).
apply is_below_avl_is_below.
apply (inv_is_avl_left k d l r b avl_t).
rewrite (equal_eq key k equal).
apply (inv_is_avl_is_is_below_avl k d l r b avl_t).
apply (lookup_above_false key r data u0).
apply is_above_avl_is_above.
apply (inv_is_avl_right k d l r b avl_t).
rewrite (equal_eq key k equal).
apply (inv_is_avl_is_is_above_avl k d l r b avl_t).
Qed.

(* Specifications: *)
Lemma lookup_avl_inv_less :
forall (key k : Int) (d : B) (l r : avl_tree) (b : bal) (data : B),
is_avl (Avl_Node k d l r b) ->
Less key k -> lookup key (Avl_Node k d l r b) data -> lookup key l data.