About the Authors

Richard J. Williams


Affiliation Microsoft Research, Cambridge, United Kingdom

Competing Interests

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.

Author Contributions

Conceived and designed the experiments: RJW. Performed the experiments: RJW. Analyzed the data: RJW. Contributed reagents/materials/analysis tools: RJW. Wrote the manuscript: RJW.