1. Department of Computer Science, Utrecht University, P.O. Box 80.089, 3508 TB Utrecht, The Netherlands
2. Department of Applied mathematics and DIMATIA, Charles University, Malostranské nám. 25, 11800 Praha 1, Czech Republic
3. Friedrich-Schiller-Universität, Fakultät für Mathematik und Informatik, 07740 Jena, Germany