Department of Computing Science, University of Glasgow, Glasgow G128QQ, UK. srogers@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow G128QQ, UK. srogers@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow G128QQ, UK. srogers@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow G128QQ, UK. srogers@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow G128QQ, UK. srogers@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow G128QQ, UK. srogers@dcs.gla.ac.uk|Department of Computing Science, University of Glasgow, Glasgow G128QQ, UK. srogers@dcs.gla.ac.uk