code: def stats(self) -> pd.DataFrame:
          
          key = ["icao24", "callsign"] if self.flight_ids is None else "flight_id"
          return (
              self.data.groupby(key)[["timestamp"]]
              .count()
              .sort_values("timestamp", ascending=False)
              .rename(columns={"timestamp": "count"})
          )
docs: Statistics about flights contained in the structure.
          Useful for a meaningful representation.