Department of Computer Science, University of Wales, Aberystwyth, Penglais, Aberystwyth, Ceredigion SY23 3DB, Wales, UK. rdk@aber.ac.uk|Department of Computer Science, University of Wales, Aberystwyth, Penglais, Aberystwyth, Ceredigion SY23 3DB, Wales, UK. rdk@aber.ac.uk|Department of Computer Science, University of Wales, Aberystwyth, Penglais, Aberystwyth, Ceredigion SY23 3DB, Wales, UK. rdk@aber.ac.uk|Department of Computer Science, University of Wales, Aberystwyth, Penglais, Aberystwyth, Ceredigion SY23 3DB, Wales, UK. rdk@aber.ac.uk