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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 add_on_results(self):
 """ Access the add_on_results :returns: twilio.rest.api.v2010.account.recording.add_on_result.AddOnResultList :rtype: twilio.rest.api.v2010.account.recording.add_on_result.AddOnResultList """

        if self._add_on_results is None:
            self._add_on_results = AddOnResultList(
                self._version,
                account_sid=self._solution['account_sid'],
                reference_sid=self._solution['sid'],
            )
        return self._add_on_results