(** * Empty map *)

Definition Empty m := forall (a:key)(e:elt) , ~ MapsTo a e m.

Lemma merge_bst : forall m1 m2, bst m1 -> bst m2 -> (forall y1 y2 : key, In y1 m1 -> In y2 m2 -> X.lt y1 y2) -> bst (merge m1 m2).