1. Sony CSL, 3- 14- 13 Higashigotanda Shinagawa-ku Tokyo 14 1-0022 Japan
2. Department of Information - Science, University of Tokyo, 7-3-1 Hongo, Bunkyoku, Tokyo 113 Japan
3. Dept. of Informatics and Mathematical Science, Osaka University, 1-3 Machikaneyama, Toyonaka Osaka 560-8531 Japan
4. Hitachi Research Lab., Hitachi, Ltd., 1 Omika-cho 7-chome, Hitachi, Ibraki, 319-1292, Japan