Department of Computer Science, University of Oxford, Oxford, U.K.|Mathematical Institute, University of Oxford, Oxford, U.K.|School of Mathematical Sciences, University of Nottingham, Nottingham, U.K.|Wellcome Trust Centre for Cell Matrix Research, University of Manchester, Manchester, U.K.|School of Biosciences, Cardiff University, Cardiff, U.K.|School of Mathematics and Statistics, University of Melbourne, Melbourne, Australia.|Department of Computer Science, University of Oxford, Oxford, U.K.|Department of Computer Science, University of Oxford, Oxford, U.K.