1. Department of Computing and Software, McMaster University, Hamilton, ON, L8S 4K1, Canada. janicki@mcmaster.ca
2. Faculty of Mathematics and Computer Science, Nicolaus Copernicus University, Toruń, Chopina 12/18, Poland and Faculty of Mathematics, Informatics and Mechanics, University of Warsaw, Warszawa, Banacha 2, Poland. lukasz.mikulski@mat.umk.pl