Center for Bioinformatics (ZBIT), Institute for Computer Science, Tübingen University, 72076 Tübingen, Germany. drichter@informatik.uni-tuebingen.de|Center for Bioinformatics (ZBIT), Institute for Computer Science, Tübingen University, 72076 Tübingen, Germany. drichter@informatik.uni-tuebingen.de|Center for Bioinformatics (ZBIT), Institute for Computer Science, Tübingen University, 72076 Tübingen, Germany. drichter@informatik.uni-tuebingen.de