1. Graduate School of Information Science and Technology Hokkaido University Kita 14, Nishi 9, Kita‐ku, Sapporo Hokkaido 060‐0814 Japan
2. Faculty of Information Science and Technology Hokkaido University Kita 14, Nishi 9, Kita‐ku, Sapporo Hokkaido 060‐0814 Japan
3. Graduate School of Science Osaka University 1‐1 Machikaneyama Toyonaka, Osaka 560‐0043 Japan