Remark 3.1.4.4. Suppose we are given a pullback diagram of simplicial sets
\[ \xymatrix@R =50pt@C=50pt{ X' \ar [r] \ar [d]^-{f'} & X \ar [d]^-{f} \\ S' \ar [r] & S. } \]
If $f$ is a covering map, then $f'$ is also a covering map.
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$