1. Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
2. Department of Computer and Information Science, University of Oregon, 1477 E. 13th Ave. Eugene, OR 97403-1202, USA
3. College of Mathematics and Informatics, Fujian Normal University, 1 Guangxian Rd, Minhou County, Fuzhou, Fujian 350117, China
4. School of Computing and Information Technology, University of Wollongong, Northfields Ave Wollongong, NSW 2522, Australia