1. Eindhoven University of Technology, Department of Mathematics and Computing Science, Den Dolech 2, 5600 MB Eindhoven, The Netherlands.
2. Microsoft Research, One Microsoft Way, Redmond, WA 98052, USA, and Eötvös Loránd University, Dept. of Computer Science, Pázmány P. sétány 1/c, 1117 Budapest, Hungary.
3. Ghent University, Dept. of Pure Maths and Computer Algebra, Krijgslaan 281—S22, 9000 Gent, Belgium.
4. Eötvös Loránd University, Dept. of Computer Science, Pázmány P. sétány 1/c, 1117 Budapest, Hungary, and Computer and Automation Institute, Hungarian Academy of Sciences, Lágymányosi u. 11, 1111 Budapest, Hungary.