Epimorphism from Polynomial Forms to Polynomial Functions

Theorem
Let $D$ be an integral domain.

Let $D \sqbrk X$ be the ring of polynomial forms in $X$ over $D$.

Let $\map P D$ be the ring of polynomial functions over $D$.

The mapping $\kappa: D \sqbrk X \to \map P D$ given by:
 * $\displaystyle \map \kappa {\sum_{k \mathop = 0}^n {a_k \circ X^k} } = f$

where $\displaystyle f = \sum_{k \mathop = 0}^n {a_k \circ x^k}, x \in D$

is a ring epimorphism.