1. Faculty of Computer Science, Electrical Engineering and Mathematics, University of Paderborn, 33102 Paderborn, Germany
2. Department of Computer Science, Clemson University, Clemson, SC 29634, USA
3. Department of Informatics, University of Bergen, N-5020 Bergen, Norway
4. Department of Computer Science, Appalachian State University, Boone, NC 28608, USA