1. Faculty of Business Administration, Hosei University, 2-17-1 Fujimi, Chiyoda-ku, Tokyo 102-8160, Japan. kurata@hosei.ac.jp
2. Department of Computer Science, Gunma University, 1-5-1 Tenjin-cho, Kiryu-shi, Gunma 376-8515, Japan. fujita@cs.gunma-u.ac.jp