CWI (Center for Mathematics and Computer Science), Kruislaan 413 and Section Computational Science, Faculty of Science, University of Amsterdam, Kruislaan 403, Amsterdam, The Netherlands. m.dobrzynski@cwi.nl|CWI (Center for Mathematics and Computer Science), Kruislaan 413 and Section Computational Science, Faculty of Science, University of Amsterdam, Kruislaan 403, Amsterdam, The Netherlands. m.dobrzynski@cwi.nl|CWI (Center for Mathematics and Computer Science), Kruislaan 413 and Section Computational Science, Faculty of Science, University of Amsterdam, Kruislaan 403, Amsterdam, The Netherlands. m.dobrzynski@cwi.nl|CWI (Center for Mathematics and Computer Science), Kruislaan 413 and Section Computational Science, Faculty of Science, University of Amsterdam, Kruislaan 403, Amsterdam, The Netherlands. m.dobrzynski@cwi.nl