Affiliations: Jan Brandts, Korteweg-de Vries Institute for Mathematics, Faculty of Science, University of Amsterdam, Science Park 105-107, 1098 XG Amsterdam, Netherlands, e-mail: J.H.Brandts@uva.nl; Michal Křížek, Institute of Mathematics, Czech Academy of Sciences, Žitná 25, 115 67 Praha 1, Czech Republic, e-mail: krizek@math.cas.cz