1. Heidelberg Institute for Theoretical Studies, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
2. Department of Computer Engineering, University of A Coruña, 15071 A Coruña, Spain
3. Department of Genetics, Evolution and Environment,University College London, Gower St., Bloomsbury, London WC1E 6BT, UK
4. Karlsruhe Institute of Technology, Department of Informatics, Institute of Theoretical Informatics, Postfach 6980, 76128 Karlsruhe, Germany