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Contributors All authors made a substantial contribution to the work. JG: in the drafting the initial effort, JG and GD: in acquiring and reviewing the images, and JG, GD, MB and ND: in manuscript review, editing and revision.
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 Nick Silver, Liverpool, UK.