$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
10.14
\begin{eqnarray} \label{equation:face-of-extra-degeneracy2} s^{n+1}_{i} \circ h_{n} & \sim & \begin{cases} h_{n+1} \circ h_{n} & \text{ if $i=0$ } \\ h_{n+1} \circ s^{n}_{i-1} & \text{ otherwise. } \end{cases}\end{eqnarray}