1. School of Knowledge Science, Japan Advanced Institute, of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan
2. Laboratory of Internet Software Technologies, Institute of Software, Chinese Academy of Sciences, Beijing 100190, P. R. China
3. Institute of Systems Science, Academy of Mathematics and Systems Science, Chinese Academy of Sciences, Beijing 100190, P. R. China