Add internal references

Thanks to  Elías Guisado
https://stacks.math.columbia.edu/tag/010I#comment-8412

homology.tex

@@ -1185,9 +1185,9 @@ \section{Extensions}
 as follows:
 \begin{enumerate}
 \item Given a morphism $B' \to B$ and an extension
-$E$ of $B$ by $A$ we define $E' = E \times_B B'$
-so that we have the following commutative diagram
-of short exact sequences
+$E$ of $B$ by $A$ we define $E' = E \times_B B'$.
+By Lemma \ref{lemma-cartesian-kernel} we have the following
+commutative diagram of short exact sequences
 $$
 \xymatrix{
 0 \ar[r] & A \ar[r] \ar[d] & E' \ar[r] \ar[d] & B' \ar[r] \ar[d] & 0 \\
@@ -1197,8 +1197,9 @@ \section{Extensions}
 The extension $E'$ is called the {\it pullback of $E$ via
 $B' \to B$}.
 \item Given a morphism $A \to A'$ and an extension
-$E$ of $B$ by $A$ we define $E' = A' \amalg_A E$
-so that we have the following commutative diagram
+$E$ of $B$ by $A$ we define $E' = A' \amalg_A E$.
+By Lemma \ref{lemma-cartesian-cocartesian}
+we have the following commutative diagram
 of short exact sequences
 $$
 \xymatrix{