# Kerodon

$\Newextarrow{\xRightarrow}{5,5}{0x21D2}$ $\newcommand\empty{}$

Warning 5.3.6.9. Definition 5.3.6.8 is not self-dual. Let $T$ be a collection of $2$-simplices of $\operatorname{\mathcal{C}}$ which satisfies the four-out-of-five property and let $T^{\operatorname{op}}$ denote the same set, regarded as a collection of $2$-simplices of the opposite simplicial set $\operatorname{\mathcal{C}}^{\operatorname{op}}$. Then $T^{\operatorname{op}}$ need not satisfy the four-out-of-five property.