Lemma 94.6.2. Let $S$ be an object of $\mathit{Sch}_{fppf}$. Consider a $2$-commutative diagram
\[ \xymatrix{ \mathcal{X}' \ar[r] \ar[d]_{f'} & \mathcal{X} \ar[d]^ f \\ \mathcal{Y}' \ar[r] & \mathcal{Y} } \]
of $1$-morphisms of categories fibred in groupoids over $(\mathit{Sch}/S)_{fppf}$. Assume the horizontal arrows are equivalences. Then $f$ is representable if and only if $f'$ is representable.
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