intros.
inversion_clear H.
left; split; trivial.
right; left; assumption.
right; right; assumption.
Qed.
(*******************************************************)
(* Lemmata about Lookup and below/above                *)

(* Specifications: *)
Lemma lookup_below_less :
forall (key : Int) (t : avl_tree) (data : B) (k0 : Int),
lookup key t data -> is_below t k0 -> Less key k0.