Remark 5.1.1.17. In the situation of Corollary 5.1.1.16, it is sufficient to verify condition $(2)$ in the special case where $B = \Delta ^ n$ is a standard simplex and $A = \operatorname{\partial \Delta }^ n$ is its boundary.
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$