1. Department of Information Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, Japan
2. AI Lab, CyberAgent, Inc., Shibuya, Tokyo, Japan
3. Graduation School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan
4. Faculty of Informatics, The University of Fukuchiyama, Fukuchiyama, Kyoto, Japan