1. Faculty of Electronics, Wrocław University of Science and Technology, Janiszewskiego 11/17, 50-372 Wrocław, Poland
2. Department of Computer Science, The University of Verona, Strada le Grazie 15, 37134 Verona, Italy
3. Department of Mathematics, Swansea University, Bay Campus, Fabian Way, Swansea SA1 8EN, UK
4. Fakultät für Mathematik, Universität Bielefeld, Postfach 110 131, 33501 Bielefeld, Germany