module BetterSet (O: Set.OrderedType) (PBLE: PPRABLE with type t = O.t): (S with type elt = O.t) =
  struct
    include Set.Make(O)
    module M = BetterMap (O) (PBLE)
    let to_map = flip (fold (flip M.add ())) M.empty
    let sdiff s1 s2 =
      M.merge
        (fun _ v1 v2 -> match v1, v2 with
          | Some (), None
          | None, Some () -> Some ()
          | _, _ -> None)
        (to_map s1)
        (to_map s2)
      |> M.keys
      |> of_enum
  end