Institute for Theoretical Computer Science, Graz University of Technology, A-8010 Graz, Austria.|Institute for Theoretical Computer Science, Graz University of Technology, A-8010 Graz, Austria.|Institute for Theoretical Computer Science, Graz University of Technology, A-8010 Graz, Austria.|Institute for Theoretical Computer Science, Graz University of Technology, A-8010 Graz, Austria.