Practical Computer Science, Faculty of Technology, Bielefeld University, Universitätsstraße 25, Bielefeld, 33615, Germany. sjanssen@techfak.uni-bielefeld.de.|Practical Computer Science, Faculty of Technology, Bielefeld University, Universitätsstraße 25, Bielefeld, 33615, Germany. robert@techfak.uni-bielefeld.de.