Department of Information Science, Graduate School of System Informatics, Kobe University, 1-1, Rokkodai, Nada, Kobe, 657-8501, Japan. thuy@iip.ist.i.kyoto-u.ac.jp.|Department of Information Science, Graduate School of System Informatics, Kobe University, 1-1, Rokkodai, Nada, Kobe, 657-8501, Japan.