1. Department of Computer Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
2. Center for Coordination Science, Sloan School of Management, Massachusetts Institute of Technology, Three Cambridge Center, NE20-336, Cambridge, MA 02142, USA
3. Department of Social Informatics, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto, 606-8501, Japan