Abstract: in Proceedings of the Sixth Workshop on General Relativity and Gravitation (The International House, Tokyo Institute of Technology, Meguro, Tokyo, December 2-5, 1996) edited by H. Ishihara, Y. Eriguchi, T. Futamase, Y. Kojima, K. Maeda, T. Nakamura, K. Nakao, Y. Nambu, M. Shibata, pp. 67-72.