(* this does not work as I expected*)
                (* this gives a warning, Lwt.return line warning: Warning 10: this expression should have type unit. *)
                readall fd >>= fun a ->
                   ignore(Lwt_io.printf "readall result\n%!");
                   ignore(Lwt_io.printf "%s\n%!" a);
                   Lwt.return(Lwt_unix.close fd ) ;


                (* this works as expected *)
                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 );) ;