1. Cardiff School of Computer Science, Cardiff University, Cardiff CF24 3AA, UK
2. Department of Mathematics, Rice University, Houston, TX 77251-1892, USA
3. Faculty of Mathematics, Karlsruhe Institute of Technology, Kaiserstrasse 12, D-76131 Karlsruhe, Germany
4. School of Mathematics, Statistics and Actuarial Sciences, University of Kent, Canterbury CT2 7NF, UK