Footnotes

• Contributors All authors jointly contributed to the ideas and argument expressed in this article. SVK wrote the first draft and MH, KS and GW subsequently revised the article.

• Competing interests All authors have completed the Unified Competing Interests form at http://www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare: no support from any organisation for the submitted work (or describe if any); no financial relationships with any organisations that might have an interest in the submitted work in the previous 3 years; no other relationships or activities that could appear to have influenced the submitted work.

• Provenance and peer review Not commissioned; externally peer reviewed.