1 Department of Actuarial Mathematics and Statistics, Heriot-Watt University, Scotland. Electronic address: denis.mollison@gmail.com.
2 Department of Statistical Science, University College London, UK. Electronic address: v.isham@ucl.ac.uk.
3 Isaac Newton Institute for Mathematical Sciences, University of Cambridge, UK. Electronic address: c.dangerfield@newton.ac.uk.
4 Big Data Institute, Li Ka Shing Centre for Health Information and Discovery, University of Oxford, UK. Electronic address: deirdre.hollingsworth@bdi.ox.ac.uk.