1. School of Computer Science and Technology, Beijing Institute of Technology, Beijing, China
2. Department of Computer and Information Science, University of Oregon, Eugene, OR, USA
3. Gesellschaft für wissenschaftliche Datenverarbeitung mbH Göttingen (GWDG), and Institute of Computer Science, University of Göttingen, Göttingen, Germany
4. Department of Computing, The Hong Kong Polytechnic University, Hong Kong