Example 3.1.1.4. Let $S$ be a simplicial set and let $S' \subseteq S$ be a simplicial subset. Then the inclusion map $S' \hookrightarrow S$ is a Kan fibration if and only if $S'$ is a summand of $S$ (see Definition 1.2.1.1).
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$