1. Faculty of Mathematics and Computer Science Jagiellonian University Łojasiewicza 6 Kraków 30‐348 Poland
2. Previous affiliation: DIMAP and Department of Computer Science University of Warwick Coventry CV4 7AL United Kingdom
3. Faculty of Informatics Masaryk University Botanická 68A Brno 602 00 Czech Republic
4. Mathematics Insitute, DIMAP and Department of Computer Science University of Warwick Coventry CV4 7AL United Kingdom
5. Department of Mathematics Massachusetts Institute of Technology Simons Building (Building 2), 77 Massachusetts Avenue Cambridge MA 02139 USA