$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
10.11
\begin{eqnarray} \label{equation:face-of-extra-degeneracy1} d^{n+1}_{i} \circ h_ n & \sim & \begin{cases} \operatorname{id}_{ X_{n} } & \text{ if $i = 0$ } \\ h_{n-1} \circ d^{n}_{i-1} & \text{ otherwise } \end{cases}\end{eqnarray}