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Contributors All authors of this letter have directly participated in its drafting. All authors of this letter have read and approved the final version submitted.
Funding The ‘Friends of the University of Liverpool’ has provided funding, to the sum of £10,000, to cover equipment costs and running costs attributable to the study.
Competing interests None.
Ethics approval This study received a favorable ethical opinion from the North Wales Research Ethics Committee—West in 2012.
Provenance and peer review Not commissioned; internally peer reviewed.