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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 asset_versions(self):
 """ Access the asset_versions :returns: twilio.rest.serverless.v1.service.asset.asset_version.AssetVersionList :rtype: twilio.rest.serverless.v1.service.asset.asset_version.AssetVersionList """

        if self._asset_versions is None:
            self._asset_versions = AssetVersionList(
                self._version,
                service_sid=self._solution['service_sid'],
                asset_sid=self._solution['sid'],
            )
        return self._asset_versions