docs: def list_documents(self, limit=None):
          """ Generates vids of all indexed identifiers.
  
          Args:
              limit (int, optional): If not empty, the maximum number of results to return
  
          Generates:
              str: vid of the document.
          """
          limit_str = ''
          if limit:
              try:
                  limit_str = 'LIMIT {}'.format(int(limit))
              except (TypeError, ValueError):
                  pass
  
          query = ('SELECT identifier FROM identifier_index ' + limit_str)
  
          for row in self.backend.library.database.connection.execute(query).fetchall():
              yield row['identifier']