1. Key Laboratory of High Confidence Software Technologies (Ministry of Education), School of Computer Science, Peking University, Beijing, China
2. School of Computer Science, Northwestern Polytechnical University, Xi'an, China
3. Key Laboratory of High Confidence Software Technologies (Ministry of Education), School of Computer Science, Peking University, Beijing, China, Telecom SudParis and Institut Polytechnique de Paris, Evry, France