1. Institute for Cognitive Systems, Department of Electrical and Computer Engineering, Technical University of Munich, Arcisstrasse 21, 80333 München, Germany.
2. Institute for Artificial Intelligence, University of Bremen, Am Fallturm 1, 28359 Bremen, Germany.
3. School of Information Science and Technology and Next Generation Artificial Intelligence Research Center, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.