1. Department of Information Engineering, Maebashi Institute of Technology, 460-Kamisadori-Cho, Maebashi-City, 371, Japan
2. Department of Computer Science, Beijing Polytechnic University, Beijing 100022, China
3. Department of Information and Computer Science, School of Science and Engineering, Waseda University, 3-4-1 Okubo Shinjuku-Ku, Tokyo 169, Japan