code: def add(self, **kwargs):
      
      new_element = self._message_descriptor._concrete_class(**kwargs)
      new_element._SetListener(self._message_listener)
      self._values.append(new_element)
      if not self._message_listener.dirty:
        self._message_listener.Modified()
      return new_element
docs: Adds a new element at the end of the list and returns it. Keyword
      arguments may be used to initialize the element.