Department of Engineering Mathematics, University of Bristol, Bristol, BS8 1TR, UK Microsoft Research, 7 J J Thomson Avenue, Cambridge, CB3 0FD, UK.|Department of Engineering Mathematics, University of Bristol, Bristol, BS8 1TR, UK Microsoft Research, 7 J J Thomson Avenue, Cambridge, CB3 0FD, UK.|Department of Engineering Mathematics, University of Bristol, Bristol, BS8 1TR, UK Microsoft Research, 7 J J Thomson Avenue, Cambridge, CB3 0FD, UK.