Footnotes

• Contributors YX conceived the article. Z-xL, JW, Z-xM, JC and Q-mQ wrote the first draft, which was critically revised by SZ and S-yZ. All authors approved the final version of the manuscript. YX is responsible for the overall content as guarantor.

• Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

• Competing interests None declared.

• Provenance and peer review Not commissioned; externally peer reviewed.