1. Facculty of Mathematics, University of Tübingen, Tübingen, 72076 Germany.
2. IMAG, Université Montpellier, CNRS, Montpellier, 34090 France.
3. ENSICAEN, Normandie Université, CNRS, GREYC, Caen, 14000 France.
4. Departments of Computer Science and Mathematics, Saarland University, Saarbrücken, 66123 Germany.