1. a Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan;, Email: syuu@cs.kumamoto-u.ac.jp
2. b Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
3. c Kumamoto National College of Technology, 2659-2 Suya, Nishigoshi 861-1102, Japan