1. CNRS, LMD, Luminy, Case 930, F-13288 Marseille Cedex 9, France
2. Universite Bordeaux I, Math., 351 cours de la Libération, F-33405 Talence Cedex, France
3. Center for Complex Systems and Visualization, University of Bremen, D-28334 Bremen, Germany
4. Department of Computer Science, University of Waterloo, Waterloo, Ontario, Canada N2G 1Y9, Canada