1. Institute for Computer Science, Goethe University Frankfurt, Robert-Mayer-Strasse 11-15, 60325 Frankfurt/Main, Germany.
2. Department of Mathematics and Computer Science, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark.
3. Center for Industrial Mathematics, University of Bremen, Bibliothekstraße 5, 28359 Bremen, Germany.