[1] Tsinghua Univ, Dept Comp Sci & Technol, Beijing 100084, Peoples R China
[2] Univ Calif Riverside, Dept Comp Sci & Engn, Riverside, CA 92521 USA
[3] Simon Fraser Univ, Sch Comp Sci, Burnaby, BC V5A 1S6, Canada
[4] Uppsala Univ, Dept Informat Technol, Uppsala, Sweden