1. Faculty of Engineering, Information and Systems, University of Tsukuba, Tsukuba, Japan
2. Department of Computer Science, University of Tsukuba, Tsukuba, Japan
3. National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan
4. Department of Statistical Science, University College London, London, U.K.
5. School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, Stockholm, Sweden