Example 4.1.5.3. Let $X$ be a simplicial set. Then the projection map $f: X \rightarrow \Delta ^0$ is an inner covering map if and only if $X$ is isomorphic to the nerve of a category (this is a restatement of Proposition 1.3.4.1).
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$