1. Department of Computer Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India;
2. Department of Industrial and Enterprise Systems Engineering, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801;
3. Department of Computer Science, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801;
4. Department of Mathematics, London School of Economics, London WC2A 2AE, United Kingdom