Affiliations: Radim Blaheta, Faculty of Electrical Engineering and Computer Science, Technical University of Ostrava, 17. listopadu 15, 708 00 Ostrava-Poruba, Czech Republic, e-mail: radim.blaheta@ugn.cas.cz; Miroslav Tůma, Faculty of Mathematics and Physics, Charles University in Prague, Sokolovská 83, 186 75 Praha 8, Czech Republic, e-mail: mirektuma@karlin.mff.cuni.cz