1. Research Center for Artificial Intelligence, Chengdu University of Technology, Chengdu 610059, P. R. China
2. School of Electrical Engineering, Southwest Jiaotong University, Chengdu 610031, P. R. China
3. COL Laboratory, School of Computer Science, University of Nottingham, Nottingham, UK
4. Department of Computer Science and Artificial Intelligence, University of Sevilla, Avda. Reina Mercedes s/n 41012, Spain