1. Microsoft Research Cambridge; presently, Research Fellow, Univ. of Southampton, SO19 7JG, United Kingdom (corresponding author).
2. Distinguished Scientist, Microsoft Research Cambridge, 7 JJ Thompston Avenue Cambridge, CB3 0FB, United Kingdom.
3. Senior Lecturer in Engineering, Dept. of Engineering, Univ. of Cambridge, Trumpington Street, Cambridge, CB2 1PZ, United Kingdom.