Footnotes

• Contributors CW was responsible for conception and design of the work. JH was responsible for the initial drafting of the article. Both authors contributed to critical revision of the article. JH reviewed and approved the final version to be published.

• 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.