[1] Indian Inst Technol, Dept Comp Sci & Engn, Madras, Tamil Nadu, India
[2] Nanyang Technol Univ, Sch Phys & Math Sci, Div Math Sci, Singapore, Singapore
[3] Univ Oxford, Dept Comp Sci, Oxford, England
[4] Univ Caen Basse Normandie, CNRS, Lab CREM, UMR 6211, F-14032 Caen, France
[5] Microsoft Res New England, Cambridge, MA USA