Department of Computing, City University, London EC1V 0HB, UK. dmb@mrc-dunn.cam.ac.uk|Department of Computing, City University, London EC1V 0HB, UK. dmb@mrc-dunn.cam.ac.uk|Department of Computing, City University, London EC1V 0HB, UK. dmb@mrc-dunn.cam.ac.uk|Department of Computing, City University, London EC1V 0HB, UK. dmb@mrc-dunn.cam.ac.uk|Department of Computing, City University, London EC1V 0HB, UK. dmb@mrc-dunn.cam.ac.uk