Department of Computer Science, University College London, Gower Street, London, WC1E 6BT, UK.|Department of Computer Science, University College London, Gower Street, London, WC1E 6BT, UK.|Department of Computer Science, University College London, Gower Street, London, WC1E 6BT, UK.|Department of Computer Science, University College London, Gower Street, London, WC1E 6BT, UK.