Unique Isomorphism from Quotient Mapping to Epimorphism Domain

Theorem
Let $\left({S, \circ}\right)$ and $\left({T, *}\right)$ be algebraic structures.

Let $\phi: \left({S, \circ}\right) \to \left({T, *}\right)$ be an epimorphism.

Let $\mathcal R_\phi$ be the equivalence induced by $\phi$.

Let $S / \mathcal R_\phi$ be the quotient of $S$ by $\mathcal R_\phi$.

Let $q_{\mathcal R_\phi}: S \to S / \mathcal R_\phi$ be the quotient mapping induced by $\mathcal R_\phi$.

Let $\left({S / \mathcal R_\phi}, {\circ_{\mathcal R_\phi}}\right)$ be the quotient structure defined by $\mathcal R_\phi$.

Then there is one and only one isomorphism:
 * $\psi: \left({S / \mathcal R_\phi}, {\circ_{\mathcal R_\phi}}\right) \to \left({T, *}\right)$

which satisfies:
 * $\psi \bullet q_{\mathcal R_\phi} = \phi$

where, in order not to cause notational confusion, $\bullet$ is used as the symbol to denote composition of mappings.

Proof
From the Quotient Theorem for Surjections, there is a unique bijection from $S / \mathcal R_\phi$ onto $T$ satisfying $\psi \bullet q_{\mathcal R_\phi} = \phi$.

Also:

Therefore $\psi$ is an isomorphism.