code: def GetNextWrittenEventSource(self):
      
      if not self._storage_file:
        raise IOError('Unable to read from closed storage writer.')
  
      event_source = self._storage_file.GetEventSourceByIndex(
          self._written_event_source_index)
      if event_source:
        self._written_event_source_index += 1
      return event_source
docs: Retrieves the next event source that was written after open.
  
      Returns:
        EventSource: event source or None if there are no newly written ones.
  
      Raises:
        IOError: when the storage writer is closed.
        OSError: when the storage writer is closed.