let foo () =
  let state = ref 3 in

  let update_event, update_send = E.create () in

  (* Note: Someone else will change state and call update_send() *)

  let update_state () =
    (* do stuff *)
    Printf.printf "State is %d\n%!" !state;
    return ()
  in

  let _ = E.keep @@ E.map_s update_state update_event in

  let main_loop () =
    lwt () = Lwt_unix.sleep 1.0 in
    main_loop ()