1. Key Laboratory of High Confidence Software Technologies (Ministry of Education), School of Electronics Engineering and Computer Science, Peking University, Beijing, P.R China
2. State Key Laboratory for Novel Software Technology, Nanjing University, Nanjing, P.R China; State Key Laboratory of Computer Sciences, Institute of Software, Chinese Academy of Sciences, Beijing, P.R China
3. Department of Computing, Macquarie University, Australia
4. School of Electronics Engineering and Computer Science, Peking University, Beijing, P.R China
5. Key Laboratory of High Confidence Software Technologies (Ministry of Education), School of Electronics Engineering and Computer Science, Peking University, Beijing, China, Département Réseaux et Services Multimédia Mobiles, Institut Polytechnique de Paris, Evry, France