(** * Empty map *)

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

Lemma concat_find : forall m1 m2 y, bst m1 -> bst m2 -> (forall y1 y2, In y1 m1 -> In y2 m2 -> X.lt y1 y2) -> find y (concat m1 m2) = match find y m2 with Some d => Some d | None => find y m1 end.