End search_tree_basic_properties.
Hint Resolve go_left go_right not_left not_right search_tree_l search_tree_r
maj_l min_r: searchtrees.
(* each key occurs at most once *)

(* Specifications: *)
Lemma occ_unicity :
forall t : btree,
search_tree t ->
forall (k : key) (v v' : data), occ v k t -> occ v' k t -> v = v'.