Institute for Theoretical Computer Science, Technische Universitaet Graz, Graz, Austria. maass@igi.tugraz.at|Institute for Theoretical Computer Science, Technische Universitaet Graz, Graz, Austria. maass@igi.tugraz.at|Institute for Theoretical Computer Science, Technische Universitaet Graz, Graz, Austria. maass@igi.tugraz.at