Department of Computing Science, University of Glasgow, Glasgow G12 8QQ, UK. srogers@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow G12 8QQ, UK. srogers@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow G12 8QQ, UK. srogers@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow G12 8QQ, UK. srogers@dcs.gla.ac.uk