Kerodon

$\Newextarrow{\xRightarrow}{5,5}{0x21D2}$
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$

Exercise 4.4.1.12. Let $X$ be a Kan complex, and let $Y \subseteq X$ be a simplicial subset. Show that $Y$ is a summand of $X$ (Definition 1.1.6.1) if and only if it is a replete full subcategory of $X$.