Graph theory: Use of undefined relation I

[ITheLestI]

Incidence relation used in Induces subgraph definition:

discrete-math-course/cheat5.tex

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	\item \textbf{Induces subgraph}\Href{https://en.wikipedia.org/wiki/Induced_subgraph} of a graph $G = \Pair{V,E}$ is another graph $G'$ formed from a subset~$S$ of the vertices of the graph and \emph{all} the edges (from the original graph) connecting pairs of vertices in that subset.
	Formally, $G' = G[S] = \Pair{V',E'}$, where $S \subseteq V$, $V' = V \intersection S$, $E' = \Set{e \in E \given \exists v \in S: e \mathrel{I} v}$.

But it was firstly defined later:

discrete-math-course/cheat5.tex

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\item \textbf{Incidence}\Href{https://en.wikipedia.org/wiki/Incidence_(geometry)} is a relation between an edge and its endpoints.