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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 credentials(self):
 """ Access the credentials :returns: twilio.rest.api.v2010.account.sip.credential_list.credential.CredentialList :rtype: twilio.rest.api.v2010.account.sip.credential_list.credential.CredentialList """

        if self._credentials is None:
            self._credentials = CredentialList(
                self._version,
                account_sid=self._solution['account_sid'],
                credential_list_sid=self._solution['sid'],
            )
        return self._credentials