Remark 9.4.6.5. Let $U: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{C}}$ be an inner fibraiton of simplicial sets. Then $U$ is flat if and only if the opposite inner fibration $U^{\operatorname{op}}: \operatorname{\mathcal{E}}^{\operatorname{op}} \rightarrow \operatorname{\mathcal{C}}^{\operatorname{op}}$ is flat.
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$