public static Flight CreateNew(ushort capacity)
 {
     return(new Flight(
                FlightId.CreateNew(),
                capacity,
                Enumerable.Empty <Booking>()));
 }
 public Flight(
     FlightId id,
     ushort capacity,
     IEnumerable <Booking> bookings)
 {
     Id        = id;
     Capacity  = capacity;
     _bookings = new List <Booking>(bookings);
 }