1. Department of Information Science, Fukui University, 3-9-1 Bunkyo, Fukui-Shi, 910-8507 Japan; e-mail: {, , , }
2. Nippon Telegraph and Telephone Corporation, Solution Business Headquarters, Nakanoshima Center Bldg., 6-2-27 Nakanoshima, Kita-ku, Osaka Pref, 530-6691, Japan;