1. Cardiff School of Computer Science and Informatics; Cardiff University; Cardiff, CF24 3AA Wales UK
2. School of Mathematics; Cardiff University; Senghennydd Road, Cardiff, CF24 4AG Wales UK
3. Department of Math. Physics; Institute of Physics, St. Petersburg State University, 1 Ulianovskaia; St. Petergoff, St. Petersburg 198504 Russia
4. School of Mathematics; Statistics and Actuarial Sciences, University of Kent; CT2 7NF Canterbury UK