1. Department of Computing and Software, McMaster University, Hamilton, ON, L8S 4K1, Canada
2. LIACS, Leiden University, P.O.Box 9512, NL-2300 RA Leiden, The Netherlands
3. School of Computing Science, Newcastle University, Newcastle upon Tyne, NE1 7RU, U.K.
4. Faculty of Mathematics and Computer Science, Nicolaus Copernicus University, Toruń, Chopina 12/18, Poland. mikulskilukasz@gmail.com