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

module IntWrapper = Univ.Embed(Int)
module StringWrapper = Univ.Embed(String)

let and_null = Hashtbl.create 16
let or_watt = Hashtbl.create 16

let name = StringWrapper.new_property ()
let age = IntWrapper.new_property ()
let money = IntWrapper.new_property ()

let get t (set, get) = get t
let set t (set, get) x = set t x

let () = set and_null name "and Null"
let () = set and_null age 28
let () = set and_null money 20
let () = set or_watt name "or Watt"
let () = set or_watt age 28
let () = set or_watt money 100