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Some of the 2-category theory articles at nlab, I think, use the semicolon for horizontal composition: i.e. the functor $\hom(Y,Z) \times \hom(X,Y) \to \hom(X,Z)$.
So, $F;U$ just means the ordinary composition of the two functors.
Except… now that I’m looking at the article it looks like it’s the reversed convention? I.e. $\hom(X,Y) \times \hom(Y,Z) \to \hom(X,Z)$, so $F;U$ means the composite $UF$?
Hi Adam, thanks very much for raising this! The page was rather a mess before, with all sorts of notational confusion and typos; I have now, as described in #3 and #4, edited the page. Hopefully things are now clear, but if not just let us know (and feel free to edit this or any other page if you see something you wish to correct or add!).
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