1. Budapest, 1111 Hungary
2. Department of Theoretical Computer Science, Université Paris Cité 8 Pl. Aurélie Nemours, 75013 Paris, France
3. Alfréd Rényi Institute of Mathematics, Hungarian Academy of Sciences Reáltanoda, utca 13-15, 1053 Budapest, Hungary
4. Department of Mathematics, IST Austria, Am Campus 1, 3400 Klosterneuburg, Austria
5. Institute of Mathematics, Eötvös Loránd University Pázmány Péter stny, 1/c, 1117 Budapest, Hungary