let rec accept_connection sock =
  let socket_thread = Lwt_unix.accept sock  in
  Lwt.bind socket_thread
           ( fun (fd, caller) ->
               ignore(Lwt_io.printf "accepted\n%!");
                (* works *)
                (*
                ignore (readall fd >>= (fun a ->
                   ignore(Lwt_io.printf "readall result\n%!");
                   ignore(Lwt_io.printf "%s\n%!" a);
                   Lwt_unix.close fd ;));
                *)
                (* works *)
                (* Warning 10: this expression should have type unit. *)
                (*
                Lwt.bind (readall fd)
                   (fun a ->
                   ignore(Lwt_io.printf "readall result\n%!");
                   ignore(Lwt_io.printf "%s\n%!" a);
                   Lwt.return(Lwt_unix.close fd );) ;
                *)
                (* does not work : *)
                (*
                readall fd >>= fun a -> Lwt_io.printf "readall result\n %s\n%!" a >>= fun () -> Lwt_unix.close fd  >>= fun () ->
                *)
               accept_connection sock;

           );;