1. EURANDOM and Department of Mathematics and Computer Science; Eindhoven University of Technology; 5600 MB Eindhoven The Netherlands
2. Department of Telecommunications and Information Processing; Ghent University; Sint-Pietersnieuwstraat 41 Ghent B9000 Belgium
3. Department of Industrial Systems Engineering and Product Design; Ghent University; Technologiepark 903 Zwijnaarde B9052 Belgium
4. Microsoft Research-Inria Joint Centre; Campus de l'Ecole Polytechnique; bâtiment Alan Turing, 1 rue Honoré d'Estienne d'Orves 91120 Palaiseau France
5. Department of Statistics; The Hebrew University of Jerusalem; Mount Scopus, Jerusalem 91905 Israel