Department of Computing Science, University of Glasgow, Glasgow, UK. jonesa@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow, UK. jonesa@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow, UK. jonesa@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow, UK. jonesa@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow, UK. jonesa@dcs.gla.ac.uk