\begin{section}{Arrow}
The identity operator on $V$ is $\identity{V}$. The composition of
$f$ and $g$ is $\compose{f}{g}$. The inverse of $f$ is
- $\inverse{f}$.
+ $\inverse{f}$. If $f$ is a function and $A$ is a subset of its
+ domain, then the preimage under $f$ of $A$ is $\preimage{f}{A}$.
\end{section}
\begin{section}{Common}
The interior of a set $X$ is $\interior{X}$. Its closure is
$\closure{X}$ and its boundary is $\boundary{X}$.
\end{section}
-
+
\end{document}