1. Department of Computer Science, Charles University, Faculty of Mathematics and Physics, Malostranské nám. 25, 118 00 PRAHA 1, Czech Republic. petercerno@gmail.com
2. Department of Computer Science, Charles University, Faculty of Mathematics and Physics, Malostranské nám. 25, 118 00 PRAHA 1, Czech Republic. mraz@ksvi.ms.mff.cuni.cz