Definition 6.1.1 (Inversion).label Let $X$ be a set and $U \subset X \times X$, then the inversion of $U$ is the set
\[U^{-1}= \bracs{(y, x)| (x, y) \in U}\]
A set $U \subset X \times X$ is symmetric if $U = U^{-1}$.
Definition 6.1.1 (Inversion).label Let $X$ be a set and $U \subset X \times X$, then the inversion of $U$ is the set
A set $U \subset X \times X$ is symmetric if $U = U^{-1}$.