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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 document_permissions(self):
 """ Access the document_permissions :returns: twilio.rest.preview.sync.service.document.document_permission.DocumentPermissionList :rtype: twilio.rest.preview.sync.service.document.document_permission.DocumentPermissionList """

        if self._document_permissions is None:
            self._document_permissions = DocumentPermissionList(
                self._version,
                service_sid=self._solution['service_sid'],
                document_sid=self._solution['sid'],
            )
        return self._document_permissions