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Contributors YHL did the literature review, writing, editing and submission of the article. AZET and SHC wrote and edited the article and provided labelled images. IA did the literature review, writing, and editing of the article.
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; internally peer reviewed.