1. Université Paris-Dauphine, PSL Research University, CNRS, UMR, 7243, LAMSADE, 75016 Paris, France.
2. University of Warsaw, 02-097 Warsaw, Poland.
3. Max Planck Institute for Informatics, Saarland Informatics Campus, 66123 Saarbrücken, Germany.
4. Royal Holloway, University of London, Egham TW20 0EX, UK.