School of Informatics, The University of Edinburgh, Edinburgh EH8 9LE, UK. stuart@aiai.ed.ac.uk|School of Informatics, The University of Edinburgh, Edinburgh EH8 9LE, UK. stuart@aiai.ed.ac.uk|School of Informatics, The University of Edinburgh, Edinburgh EH8 9LE, UK. stuart@aiai.ed.ac.uk|School of Informatics, The University of Edinburgh, Edinburgh EH8 9LE, UK. stuart@aiai.ed.ac.uk