Department of Computer Science, University of Wales, Aberystwyth, Penglais, UK. jat@aber.ac.uk|Department of Computer Science, University of Wales, Aberystwyth, Penglais, UK. jat@aber.ac.uk|Department of Computer Science, University of Wales, Aberystwyth, Penglais, UK. jat@aber.ac.uk|Department of Computer Science, University of Wales, Aberystwyth, Penglais, UK. jat@aber.ac.uk