1. Faculty of Management and Information ScienceJosai International UniversityGumyo, ToganeChiba283‐0002Japan
2. Graduate School of Engineering, Kyushu Institute of TechnologySensui 1‐1TobataKitakyushu804‐8550Japan
3. Department of InformaticsTokyo University of Information Sciences4‐1 OnaridaiWakaba‐kuChiba265‐8501Japan