def extend(self, elem_seq):
    """Extends by appending the given sequence of elements of the same type
    as this one, copying each individual message.
    """
    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()
Extends by appending the given sequence of elements of the same type
    as this one, copying each individual message.