code: def _check_dep_time_is_valid(self, dep_time):
          
          assert dep_time <= self._min_dep_time, "Labels should be entered in decreasing order of departure time."
          dep_time_index = self.dep_times_to_index[dep_time]
          if self._min_dep_time < float('inf'):
              min_dep_index = self.dep_times_to_index[self._min_dep_time]
              assert min_dep_index == dep_time_index or (min_dep_index == dep_time_index - 1), \
                  "dep times should be ordered sequentially"
          else:
              assert dep_time_index is 0, "first dep_time index should be zero (ensuring that all connections are properly handled)"
          self._min_dep_time = dep_time
docs: A simple checker, that connections are coming in descending order of departure time
          and that no departure time has been "skipped".
  
          Parameters
          ----------
          dep_time
  
          Returns
          -------
          None