1. John Innes Centre, Norwich Research Park, Norwich NR4 7UH, UK.
2. School of Computing Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK.
3. Department of Computer Science, University of Calgary, Calgary, Alberta T2N 1N4, Canada.
4. MRC Centre for Developmental and Biomedical Genetics and Department of Biomedical Science, University of Sheffield, Sheffield S10 2TN, UK.