docs: def _add_source(model):
      """
      Go over all attributes in `model` and add :class:`SourceString` to them.
  
      Args:
          model (obj): :class:`Model` instance.
  
      Returns:
          obj: :class:`Model` instance with :class:`SourceString` descriptors.
      """
      ignored_keys = {"author_tags", "original_xml", "additional_info"}
  
      # convert all values to source strings
      source = "Aleph"
      for key, val in model.get_mapping().iteritems():
          if key in ignored_keys:
              continue
  
          if type(val) in [list, tuple]:
              ss_val = [
                  SourceString(item, source).to_dict()
                  for item in val
              ]
          else:
              ss_val = [SourceString(val, source).to_dict()]
  
          setattr(model, key, ss_val)
  
      return model