1. Department of Actuarial Mathematics and Statistics, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom;
2. Mathematics Institute, University of Warwick, Coventry CV4 7AL, United Kingdom;
3. Computer Laboratory, University of Cambridge, Cambridge CB3 0FD, United Kingdom;
4. Department of Actuarial Mathematics and Statistics, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom