$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
7.72
\begin{equation} \begin{gathered}\label{equation:sequential-limit-as-equalizer} \xymatrix@R =50pt@C=50pt{ P \ar [r]^-{ [f] } \ar [d]^-{ [ q_{n+1} ]} & P \ar [d]^-{ [q_ n] } \\ X(n+1) \ar [r]^-{ [f_ n] } & X(n) } \end{gathered} \end{equation}