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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 public_key(self):
 """ Access the public_key :returns: twilio.rest.accounts.v1.credential.public_key.PublicKeyList :rtype: twilio.rest.accounts.v1.credential.public_key.PublicKeyList """

        if self._public_key is None:
            self._public_key = PublicKeyList(self._version, )
        return self._public_key