1. Graduate School of Information Science and Technology Hokkaido University Kita 14, Nishi 9, Kita‐ku, Sapporo Hokkaido 060‐0814 Japan
2. Graduate School of Natural Science and Technology Okayama University 3‐1‐1, Tsushima‐naka, Kita‐ku Okayama 700‐8530 Japan
3. Toshiba Mitsubishi‐Electric Industrial Systems Corporation 3‐1‐1, Kyobashi, Chuo‐ku Tokyo 104‐0031 Japan