Department of Information and Communication Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan;Department of Information and Communication Engineering, IBM Research, Tokyo Research Laboratory, 1623-14 Shimo-tsuruma, Yamato, Kanagawa, Japan;Department of Information and Communication Engineering, National Institute of Informatics, 1623-14 Shimo-tsuruma, Yamato, Tokyo, Japan