School of Informatics, University of Edinburgh, 11 Crichton Street, Edinburgh, UK. stuart@aiai.ed.ac.uk|School of Informatics, University of Edinburgh, 11 Crichton Street, Edinburgh, UK. stuart@aiai.ed.ac.uk|School of Informatics, University of Edinburgh, 11 Crichton Street, Edinburgh, UK. stuart@aiai.ed.ac.uk