Centre for Mathematics and Computer Science, CWI, Kruislaan 413, Amsterdam, SJ, The Netherlands;Leiden Institute of Advanced Computer Science, Leiden University, Kruislaan 413, Leiden, RA, The Netherlands;Centre Equitation, VERIMAG, 2 Avenue de Vignate, Grenoble-Gières, RA, France;Institute of Computer Science and Applied Mathematics, Christian-Albrechts University Kiel, Hermann-Rodewald-Straße 3, Kiel, RA, Germany