1. Research and Development Group, Hitachi, Ltd, Yoshida-cho 292, Yokohama-shi, Kanagawa, 244–0817, Japan
2. School of Computing, Tokyo Institute of Technology, Ookayama 2–12–1, Meguro-ku, Tokyo 152–8552, Japan
3. Faculty of Computer Science, Østfold University College, NO–1757 Halden, Norway