Graduate School of System Informatics, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan. kyogoku@cs25.scitec.kobe-u.ac.jp|Graduate School of System Informatics, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan. kyogoku@cs25.scitec.kobe-u.ac.jp|Graduate School of System Informatics, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan. kyogoku@cs25.scitec.kobe-u.ac.jp|Graduate School of System Informatics, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan. kyogoku@cs25.scitec.kobe-u.ac.jp