1. Computer and Automation Research Institute, Hungarian Academy of Sciences, H-1111 Budapest, Kende u. 13-17, Hungary. csuhaj@sztaki.hu
2. Institute of Mathematics, Czech Academy of Sciences, Žižkova 22, 61662 Brno, Czech Republic. masopust@ipm.cz
3. Computer and Automation Research Institute, Hungarian Academy of Sciences, H-1111 Budapest, Kende u. 13-17, Hungary. vaszil@sztaki.hu