code: def extend(self, elem_seq):
      
      message_class = self._message_descriptor._concrete_class
      listener = self._message_listener
      values = self._values
      for message in elem_seq:
        new_element = message_class()
        new_element._SetListener(listener)
        new_element.MergeFrom(message)
        values.append(new_element)
      listener.Modified()
docs: Extends by appending the given sequence of elements of the same type
      as this one, copying each individual message.