Footnotes

• Contributors DRT wrote the first draft and SSP contributed original material to the manuscript and provided revisions. DRT finalised the article.

• 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; internally peer reviewed.

• Author note Author note References which include a * are considered to be key references.