Department of Computer Science, University College London, Gower Street, London WC1E 6BT, UK. l.mcguffin@cs.ucl.ac.uk|Department of Computer Science, University College London, Gower Street, London WC1E 6BT, UK. l.mcguffin@cs.ucl.ac.uk|Department of Computer Science, University College London, Gower Street, London WC1E 6BT, UK. l.mcguffin@cs.ucl.ac.uk|Department of Computer Science, University College London, Gower Street, London WC1E 6BT, UK. l.mcguffin@cs.ucl.ac.uk