1. Department of Computing Science, Carl von Ossietzky Universität Oldenburg, Germany. E-mails: fraenzle@informatik.uni-oldenburg.de, herde@informatik.uni-oldenburg.de, teige@informatik.uni-oldenburg.de
2. Institute of Computer Science, Academy of Sciences of the Czech Republic, Prague, Czech Republic. E-mail: stefan.ratschan@cs.cas.cz
3. Faculty of Applied Sciences, Albert-Ludwigs-Universität Freiburg, Germany. E-mail: schubert@informatik.uni-freiburg.de