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