1. Institute for Information and Communication Technologies, Electronics and Applied Mathematics, Université Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium;
2. Department of Industrial Engineering and Management Sciences, Northwestern University, Evanston, Illinois 60208;
3. PSR, 22250-040 Rio de Janeiro-RJ, Brazil;
4. Pontifícia Universidade Católica do Rio de Janeiro, 22451-900 Rio de Janeiro-RJ, Brazil;
5. Google Research, New York, New York 10011