\begin{definition}[Definition:Nest]
Let $S$ be a set.
Let $\powerset S$ be its power set.
Let $N \subseteq \powerset S$ be a subset of $\powerset S$.
Then $N$ is a '''nest''' {{iff}}:
:$\forall X, Y \in N: X \subseteq Y$ or $Y \subseteq X$
\end{definition}