1. Institute for Software Technology, University of Technology, Inffeldgasse 16B/II, 8010 Graz, Austria
2. Institute for Theoretical Computer Science, University of Technology, Inffeldgasse 16B/II, 8010 Graz, Austria
3. Institute of Applied Geometry, Johannes Kepler University, Altenberger Str. 69, 4040 Linz, Austria
4. Faculty of Mathematics and Physics, Charles University in Prague, Sokolovská 83, 186 75 Praha 8, Czech Republic