2021-07-01: Aligning \qedhere after TikZ diagram             rak
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Getting the QED tomb properly aligned with the bottom of a
diagram can be annoying. Especially when you have done it in the
past, but have forgotten how. To help future me out, the
solution is to wrap the diagram in a gathered environment with
bottom alignment, and then to add a magic new lign right after
you close the (in this case) tikzcd environment:

\begin{gathered}[b]
\begin{tikzcd}
FX
\ar[r, "{\eta_X}"]
\ar[d, swap, "Ff"]
&
GX
\ar[d, "Gf"]
\\
FY
\ar[r, "{\eta_Y}"]
&
GY
\end{tikzcd}\\[-\dp\strutbox]
\end{gathered}\qedhere