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