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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 assigned_add_ons(self):
 """ Access the assigned_add_ons :returns: twilio.rest.api.v2010.account.incoming_phone_number.assigned_add_on.AssignedAddOnList :rtype: twilio.rest.api.v2010.account.incoming_phone_number.assigned_add_on.AssignedAddOnList """

        if self._assigned_add_ons is None:
            self._assigned_add_ons = AssignedAddOnList(
                self._version,
                account_sid=self._solution['account_sid'],
                resource_sid=self._solution['sid'],
            )
        return self._assigned_add_ons