\begin{definition}[Definition:Crossing (Jordan Curve)]
Let $P$ be a polygon embedded in $\R^2$.
Let $q \in \R^2 \setminus \partial P$.
Let $\mathbf v \in R^2 \setminus \set {\mathbf 0}$ be a non-zero vector.
Let $\LL = \set {q + s \mathbf v: s \in \R_{\ge 0} }$ be a ray with start point $q$.
Then $\LL \cap \partial P$ consists of a finite number of line segments, where $\partial P$ denotes the boundary of $P$.
As two adjacent sides in $P$ do not form a straight angle by the definition of polygon, each line segment is either a single point or an entire side of $P$.
Each of these line segments is called a '''crossing''' {{iff}} the line segment is one of these:
:a single point which is not a vertex of $P$
:a single vertex of $P$, and its adjacent sides lie on opposite sides of $\LL$
:a side $S$ of $P$, and the two sides adjacent to $S$ lie on opposite sides of $\LL$.
\end{definition}