Institute for Theoretical Computer Science, Graz University of Technology, Graz, Austria. nessler@igi.tugraz.at|Institute for Theoretical Computer Science, Graz University of Technology, Graz, Austria. nessler@igi.tugraz.at|Institute for Theoretical Computer Science, Graz University of Technology, Graz, Austria. nessler@igi.tugraz.at|Institute for Theoretical Computer Science, Graz University of Technology, Graz, Austria. nessler@igi.tugraz.at