(** * Join *)

(* Specifications: *)
Lemma join_avl_1 : forall l x d r, avl l -> avl r ->
avl (join l x d r) /\
0<= height (join l x d r) - max (height l) (height r) <= 1.
Lemma join_avl : forall l x d r, avl l -> avl r -> avl (join l x d r).
#[local]
Hint Resolve join_avl : core.