\begin{definition}[Definition:Relation Segment]
Let $X$ be a set.
Let $\RR$ be a relation on $X$.
Let $x \in X$.
Then the '''$\RR$-segment of $x$''', denoted by $x^\RR$, is defined by:
:$x^\RR := \set {y \in X: \tuple {y, x} \in \RR}$
\end{definition}