docs: def convert_identifiers(self, identifiers: Union[Identifier, List[Identifier]]):
          """
          Convert an individual :class:`Identifier` to a model instance,
          or a list of Identifiers to a list of model instances.
          """
          if not identifiers:
              return identifiers
  
          def _create_or_update(identifier):
              data = self._cache[identifier.key]
              return self.factory.create_or_update(identifier, data)[0]
  
          if isinstance(identifiers, Identifier):
              return _create_or_update(identifiers)
          elif isinstance(identifiers, list) and isinstance(identifiers[0], Identifier):
              return [_create_or_update(identifier) for identifier in identifiers]
          else:
              raise TypeError('`identifiers` must be an Identifier or list of Identifiers.')