$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
1.29
\begin{equation} \label{equation:monoids-as-semisimplicial-objects} s^{n}_ i( \sigma _{x_{n}, \cdots , x_1} ) = \sigma _{ x_ n, \cdots , x_{i+1}, e, x_ i, \cdots , x_1}. \end{equation}