1. Department of Electrical Engineering and Computer Science, Technical University of Berlin, Franklinstrasse 28/29, D-10587 Berlin, Germany
2. Department of Computer Science, University of Bern, Neubrüuckstrasse 10, CH-3012 Bern, Switzerland
3. Department of Computer Science, Attila Jozsef University, Arpad tér 2, H-6720 Szeged, Hungary