Example 1.1.1.6. Let $S_{\bullet }$ be a simplicial set and let $e \in S_{\bullet }$ be an edge of $S_{\bullet }$. Then $s = d^{1}_{1}(e)$ is a vertex of $S_{\bullet }$ which we refer to as the source of $e$, and $t = d^{1}_{0}(e)$ is a vertex of $S_{\bullet }$ which we refer to as the target of $e$. We will sometimes write $e: x \rightarrow y$ to indicate that $e$ is an edge of $S_{\bullet }$ having source vertex $x$ and target vertex $y$.
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