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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 _to_pb(self):
 """ Create cluster proto buff message for API calls """

        client = self._instance._client
        location = client.instance_admin_client.location_path(
            client.project, self.location_id
        )
        cluster_pb = instance_pb2.Cluster(
            location=location,
            serve_nodes=self.serve_nodes,
            default_storage_type=self.default_storage_type,
        )
        return cluster_pb