Department of Computer Science, Heriot-Watt University, Edinburgh, UK. kjs1@hw.ac.uk|Department of Computer Science, Heriot-Watt University, Edinburgh, UK. kjs1@hw.ac.uk|Department of Computer Science, Heriot-Watt University, Edinburgh, UK. kjs1@hw.ac.uk|Department of Computer Science, Heriot-Watt University, Edinburgh, UK. kjs1@hw.ac.uk