About the Authors

Li Zhaoping

z.li@ucl.ac.uk

Affiliation University College London, London, United Kingdom

Li Zhe

Affiliation Tsinghua University, Beijing, China

Competing Interests

The authors have declared that no competing interests exist.

Author Contributions

Analyzed the data: LZhe LZha. Wrote the paper: LZha. Conceived and designed the project: LZha.