$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
4.53
\begin{equation} \begin{gathered}\label{equation:homotopy-pullback-square-of-morphism-spaces} \xymatrix@R =50pt@C=50pt{ \operatorname{Hom}_{\operatorname{\mathcal{C}}}(X,Y)_{e} \ar [r] \ar [d] & \operatorname{Hom}_{\operatorname{\mathcal{C}}}(X,Y) \ar [d] \\ \{ e\} \ar [r] & \operatorname{Hom}_{\operatorname{\mathcal{D}}}( q(X), q(Y) ). } \end{gathered} \end{equation}