1. ICTEAM, Université catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium
2. naXys and Department of Mathematics, University of Namur, B-5000 Namur, Belgium
3. Center for International Development, Harvard University, Cambridge, Maasachusetts 02138, USA
4. Computation and Neural Systems Program, California Institute of Technology, Pasadena, California 91125, USA
5. CORE, Université catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium
6. Department of Mathematics, Imperial College London, London SW7 2AZ, United Kingdom