+ \begin{section}{Set theory}
+ Here's a set $\set{1,2,3} = \setc{n \in \Nn[1]}{ n \le 3 }$. The
+ cardinality of the set $X \coloneqq \set{1,2,3}$ is $\card{X} =
+ 3$, and its powerset is $\powerset{X}$.
+
+ We also have a few basic set operations, for example the union of
+ two or three sets: $\union{A}{B}$, $\unionthree{A}{B}{C}$. And of
+ course with union comes intersection: $\intersect{A}{B}$,
+ $\intersectthree{A}{B}{C}$. The Cartesian product of two sets $A$
+ and $B$ is there too: $\cartprod{A}{B}$. If we take the product
+ with $C$ as well, then we obtain $\cartprodthree{A}{B}{C}$.
+
+ We can also take an arbitrary (indexed) union, intersection, or
+ Cartesian product of things, like
+ $\unionmany{k=1}{\infty}{A_{k}}$,
+ $\intersectmany{k=1}{\infty}{B_{k}}$, or
+ $\cartprodmany{k=1}{\infty}{C_{k}}$. The best part about those is
+ that they do the right thing in a display equation:
+ %
+ \begin{equation*}
+ \unionmany{k=1}{\infty}{A_{k}}
+ \ne
+ \intersectmany{k=1}{\infty}{B_{k}}
+ \ne
+ \cartprodmany{k=1}{\infty}{C_{k}}.
+ \end{equation*}
+ %
+ \end{section}
+