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Contributors GP contributed to conception and design, drafting and critical revision of the article, and collection of images. DPM contributed to conception and design, drafting and critical revision of the article, and collection of images. Both authors approved the final draft.
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.