$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
6.15
\begin{equation} \begin{gathered}\label{equation:universal-localization-pushout} \xymatrix@R =50pt@C=50pt{ X_{01} \ar [dr]^{ f_{01} } \ar [rr] \ar [dd] & & X_{0} \ar [dd] \ar [dr]^{ f_{0} } & \\ & S_{01} \ar [rr] \ar [dd] & & S_{0} \ar [dd] \\ X_{1} \ar [rr] \ar [dr]^{f_1} & & X \ar [dr]^{f} & \\ & S_{1} \ar [rr] & & S } \end{gathered} \end{equation}