1. Department of Computer Science, University of Saskatchewan, 110 Science Place, Saskatoon, SK S7N 5C9, Canada
2. Mathematical Institute, Slovak Academy of Sciences, Grešákova 6, 040 01 Košice, Slovakia
3. Institute of Computer Science, Faculty of Science, P. J. Šafárik University, Jesenná 5, 040 01 Košice, Slovakia