Footnotes

• Contributors RD and JWC discussed the approach and concept of the paper; RD wrote the first draft and revisions, incorporating JWC’s input. Both authors approved the final version.

• 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 Commissioned; externally peer reviewed.