$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
7.59
\begin{equation} \begin{gathered}\label{equation:exact-sequence-pullback-square} \xymatrix@C =50pt@R=50pt{ B_{\bullet }H \ar [r] \ar [d] & B_{\bullet } G \ar [d] \\ \Delta ^0 \ar [r] & B_{\bullet }(G/H) } \end{gathered} \end{equation}