code: private Billing generateDefaultBillingWithReservation() {
          Billing billing = new Billing();
          billing.withReservation(new Reservation().withReservationLength(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