1. Formerly at Department of Mathematics and Mathematical statistics, Umeå University, SE-901 87 Umeå, Sweden.
2. Institute for Applied Mathematics & Interdisciplinary Center for Scientific Computing (IWR), Heidelberg University, D-69120 Heidelberg, Germany.
3. Institute of Mathematics, University of Mannheim, D-68138 Mannheim, Germany.
4. Clinical Operational Research Unit, Department of Mathematics, University College London, London WC1H OBT, UK.