Module MP:= WPropertiesOn U M.Definition mkEns : M.t -> Ensemble M.elt :=
   fun s x => M.In x s.Notation " !! " := mkEns.

(* Specifications: *)
Lemma In_In : forall s x, M.In x s <-> In _ (!!s) x.
Lemma Subset_Included : forall s s', s[<=]s' <-> Included _ (!!s) (!!s').