(** * join *)

(* Specifications: *)
Lemma join_in : forall l x d r y,
In y (join l x d r) <-> X.eq y x \/ In y l \/ In y r.
Lemma join_bst : forall l x d r, bst l -> bst r ->
lt_tree x l -> gt_tree x r -> bst (join l x d r).