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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 certificates(self):
 """ Access the certificates :returns: twilio.rest.preview.deployed_devices.fleet.certificate.CertificateList :rtype: twilio.rest.preview.deployed_devices.fleet.certificate.CertificateList """

        if self._certificates is None:
            self._certificates = CertificateList(self._version, fleet_sid=self._solution['sid'], )
        return self._certificates