Example 6.15.5. The lemma will be applied often to the following situation. Suppose that we have a diagram
\xymatrix{ A \ar[r] & B \ar[d] \\ C \ar[r] & D }
in \mathcal{C}. Suppose C \to D is injective on underlying sets, and suppose that the composition A \to B \to D has image on underlying sets in the image of C \to D. Then we get a commutative diagram
\xymatrix{ A \ar[r] \ar[d] & B \ar[d] \\ C \ar[r] & D }
in \mathcal{C}.
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