Department of Computer Science, University of Wales, Aberystwyth, Penglais, Aberystwyth, Ceredigion, Wales, UK. mcr04@aber.ac.uk <mcr04@aber.ac.uk>|Department of Computer Science, University of Wales, Aberystwyth, Penglais, Aberystwyth, Ceredigion, Wales, UK. mcr04@aber.ac.uk <mcr04@aber.ac.uk>|Department of Computer Science, University of Wales, Aberystwyth, Penglais, Aberystwyth, Ceredigion, Wales, UK. mcr04@aber.ac.uk <mcr04@aber.ac.uk>