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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 verification_checks(self):
 """ Access the verification_checks :returns: twilio.rest.preview.acc_security.service.verification_check.VerificationCheckList :rtype: twilio.rest.preview.acc_security.service.verification_check.VerificationCheckList """

        if self._verification_checks is None:
            self._verification_checks = VerificationCheckList(self._version, service_sid=self._solution['sid'], )
        return self._verification_checks