1. Department of Mathematics and the Maxwell Institute for Mathematical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
2. Department of Computer Science, University of Bath, Bath BA2 7AY, UK
3. Faculty of Sciences and Technology, Department of Mathematics, Meijo University, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya 468-8502, Japan