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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def current_arg_text(self) -> str: """ Plain text part in the current argument, without any CQ codes. """

        if self._current_arg_text is None:
            self._current_arg_text = Message(
                self.current_arg).extract_plain_text()
        return self._current_arg_text