1. a Computer Science and Artificial Intelligence Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02129, USA;, Email: ryuma@csail.mit.edu
2. b Graduate School of Interdisciplinary Information Studies, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
3. c Graduate School of Information Science and Technology, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan