code: private Billing generateDefaultReservation() {
          Billing billing = new Billing();
          Billing.Reservation reservation = new Billing.Reservation();
          billing.setReservation(reservation);
          reservation.setReservationLength(1);
          return billing;
      }
docs: The method to generate a default Billing with default Reservation which default ReservationLength is 1.
  
  @return The Billing object with default Reservation which default ReservationLength is 1