Exercise 4.4.1.13. Let $X$ be a Kan complex, and let $Y \subseteq X$ be a simplicial subset. Show that $Y$ is a summand of $X$ (Definition 1.2.1.1) if and only if it is a replete full subcategory of $X$.
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$