code: def GetMerger(self, cls):
      
      for merger in self._mergers:
        if isinstance(merger, cls):
          return merger
      raise LookupError('No matching DataSetMerger found')
docs: Looks for an added DataSetMerger derived from the given class.
  
      Args:
        cls: A class derived from DataSetMerger.
  
      Returns:
        The matching DataSetMerger instance.
  
      Raises:
        LookupError: No matching DataSetMerger has been added.