code: def destination(self) -> Optional[Union[str, Set[str]]]:
          
          if "destination" not in self.data.columns:
              return None
          tmp = set(self.data.destination)
          if len(tmp) == 1:
              return tmp.pop()
          logging.warn("Several destinations for one flight, consider splitting")
          return tmp
docs: Returns the unique destination value(s) of the DataFrame.
  
          The destination airport is mostly represented as a ICAO or a IATA code.