Add level edge definition to plane depth sequencing paper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

papers/plane_depth_sequencing/paper.tex

@@ -86,6 +86,10 @@ Let $G$ be a graph with a plane embedding, and let $C$ be the outer cycle of tha
 where $d(v, u)$ denotes the graph distance between $v$ and $u$ in $G$.
 \end{definition}
 
+\begin{definition}
+An edge $\{u, v\} \in E(G)$ is a \emph{level edge} if $\mathrm{depth}(u) = \mathrm{depth}(v)$.
+\end{definition}
+
 \begin{definition}
 Let $G$ be a maximal planar graph with a plane embedding and outer cycle $C$. The \emph{deep embedding} of $G$ is the graph $G'$ obtained from $G$ by the following operation: for every 3-cycle $\{u, v, w\} \subseteq V(G)$ such that
 \[