1. School of Computer Science, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada
2. Departament de Matemàtica Aplicada II, Universitat Politècnica de Catalunya Edifici Omega, Campus Nord, Jordi Girona 1-3, 08034 Barcelona, Spain
3. Department of Computer Science and Engineering, Polytechnic Institute of New York University, 5 MetroTech Center, Brooklyn, NY 11201, USA
4. Maître de recherches du F.R.S.-FNRS. Département d'Informatique, Université Libre de Bruxelles, ULB CP212, 50 Av. F.D. Roosevelt, 1050 Brussels, Belgium
5. Science Department, Roosevelt Academy, P. O. Box 94, NL-4330 AB Middelburg, The Netherlands
6. Departament de Matemàtica Aplicada II, Universitat Politècnica de Catalunya, Edifici Omega, Campus Nord, Jordi Girona 1-3, 08034 Barcelona, Spain
7. Department of Applied Mathematics, Charles University Malostranské nám. 25, 118 00 Praha 1, Czech Republic
8. Department of Mathematics and Statistics, The University of Melbourne, Australia