CeVis, University of Bremen, Universitaetsallee 29, 28359 Bremen, Germany. koenig@cevis.uni-bremen.de <koenig@cevis.uni-bremen.de>|CeVis, University of Bremen, Universitaetsallee 29, 28359 Bremen, Germany. koenig@cevis.uni-bremen.de <koenig@cevis.uni-bremen.de>