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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 toll_free(self):
 """ Access the toll_free :returns: twilio.rest.api.v2010.account.incoming_phone_number.toll_free.TollFreeList :rtype: twilio.rest.api.v2010.account.incoming_phone_number.toll_free.TollFreeList """

        if self._toll_free is None:
            self._toll_free = TollFreeList(self._version, account_sid=self._solution['account_sid'], )
        return self._toll_free