Louise Ward

Footnotes Handling editor Patrick J Twomey. Contributors JAT and WW contributed to the conception and design of the paper and wrote the first draft, which has been edited and approved by all authors for the final version of the 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 Commissioned; externally peer reviewed.