docs: def system_add_column_family(self, cf_def):
      """
      adds a column family. returns the new schema id.
  
      Parameters:
       - cf_def
      """
      self._seqid += 1
      d = self._reqs[self._seqid] = defer.Deferred()
      self.send_system_add_column_family(cf_def)
      return d