Footnotes

• Contributors XXZ and HJJ conceived of the study and drafted the manuscript. XXZ participated in its design and coordination and helped to draft the manuscript. Both authors read and approved the final manuscript.

• 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; internally peer reviewed.