module Univ = struct
    type t = ..
    let id =
        let id = ref 0 in
        fun () ->
            incr id ;
            !id

    module Embed (M : sig type t end) = struct
        type t += E of M.t
        let wrap m = E m
        let read = function E m -> Some m | _ -> None

        let new_property () =
            let id = id () in
            let set t v =
                Hashtbl.replace t id @@ wrap v in
            let get t =
                Hashtbl.find_opt t id |> Fun.flip Option.bind read in
            (set, get)
    end
end