Department of Computer Science, University of Leipzig, Leipzig, Germany. steffi@bioinf.uni-leipzig.de|Department of Computer Science, University of Leipzig, Leipzig, Germany. steffi@bioinf.uni-leipzig.de|Department of Computer Science, University of Leipzig, Leipzig, Germany. steffi@bioinf.uni-leipzig.de|Department of Computer Science, University of Leipzig, Leipzig, Germany. steffi@bioinf.uni-leipzig.de