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Contributors MRB conceived the article. MRB and TLW performed the literature search. MRB wrote the first draft of the article. TLW and DB reviewed, critiqued and edited the article. MRB as guarantor accepts full responsibility for the work and was responsible for the decision to publish.
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. Externally peer reviewed by Martin Turner, Oxford, UK, and Colin Mumford, Edinburgh, UK.