1. Department of Computer Science, Concordia University, 1455 Boulevard de Maisonneuve O., Montréal Québec, H3G 1M8, Canada
2. Department of Applied Mathematics and Computer Science, Ghent University, Krijgslaan 281-S9, B-9000 Gent, Belgium
3. Mathematisch Instituut, Universiteit Utrecht, Boedapestlaan 6, 3584 CD Utrecht, The Netherlands