Liu Dongmei Chinese Acad Sci, Natl Engn Res Ctr Fundamental Software, Inst Software, Beijing 100080, Peoples R China. Qing Sihan Chinese Acad Sci, Engn Res Ctr Informat Secur Technol, Inst Software, Beijing 100080, Peoples R China. Liu Dongmei Chinese Acad Sci, Grad Sch, Beijing 100039, Peoples R China.