docs: def list_documents(self, limit=None):
          """
          Return a list of the documents
          :param limit:
          :return:
          """
          from itertools import chain
  
          return chain(self.backend.dataset_index.list_documents(limit=limit),
                       self.backend.partition_index.list_documents(limit=limit),
                       self.backend.identifier_index.list_documents(limit=limit))