About the Author
- Richard J. Williams
- Microsoft Research, Cambridge, United Kingdom
The author was employed by Microsoft Research when this study was done. This does not affect the authors' ability to comply with PLoSONE's policies on sharing data and materials.
Conceived and designed the experiments: RJW. Performed the experiments: RJW. Analyzed the data: RJW. Contributed reagents/materials/analysis tools: RJW. Wrote the manuscript: RJW.