1. School of Information Science, Japan Advanced Institute of Science and Technology, Nomi 923-1211, Ishikawa, Japan
2. Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, Japan
3. Division of Computer Science, University of Aizu, 90 Kamiiawase, Aizuwakamatsu, Fukushima, Japan