Institute for Computer Science, Ludwig-Maximilians-University, Munich, Germany. kb@dbs.ifi.lmu.de|Institute for Computer Science, Ludwig-Maximilians-University, Munich, Germany. kb@dbs.ifi.lmu.de|Institute for Computer Science, Ludwig-Maximilians-University, Munich, Germany. kb@dbs.ifi.lmu.de|Institute for Computer Science, Ludwig-Maximilians-University, Munich, Germany. kb@dbs.ifi.lmu.de|Institute for Computer Science, Ludwig-Maximilians-University, Munich, Germany. kb@dbs.ifi.lmu.de|Institute for Computer Science, Ludwig-Maximilians-University, Munich, Germany. kb@dbs.ifi.lmu.de