1. Department of Mathematics, Loughborough University, England, and Institute of Computational and Experimental Research in Mathematics, 121 South Main Street, Providence, RI 02903, USA
2. Korteweg-de Vries Instituut, Universiteit van Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands, and Université du Luxembourg, Unité de Recherche en Mathématiques, L-4364 Esch-sur-Alzette, Luxembourg