Epic Equalizer is Isomorphism

Theorem
Let $\mathbf C$ be a metacategory.

Let $e: E \to C$ be the equalizer of two morphisms $f, g: C \to D$.

Let $e$ be an epimorphism.

Then $e$ is an isomorphism.

Proof
As $e$ equalises $f$ and $g$, $f \circ e = g \circ e$.

Since $e$ is epic, it follows that $f = g$.

Then in the equaliser diagram:


 * $\begin{xy}\xymatrix{

E  \ar[r]^*{e} & A \ar[r] ^*{f} \ar[r]<-2pt>_*{g} & B

\\ A \ar@{.>}[u]^*{k} \ar[ur]_*{\operatorname{id}_A} }\end{xy}$

We have that:
 * $f \circ 1_A = g \circ 1_A$

so there is a unique $k$ with $e \circ k = \operatorname{id}_A$.

Then:

By Equalizer is Monomorphism, it follows that $k \circ e = \operatorname{id}_E$.

This gives $k$ as an inverse to $e$.

Thus $e$ is an isomorphism.