Let $F$ be a field and let
$$
\begin{aligned}
&I=\left\{a_{n} x^{n}+a_{n-1} x^{n-1}+\cdots+a_{0} \mid a_{n}, a_{n-1}, \ldots, a_{0} \in F\right. \text { and } \\
&\left.\quad a_{n}+a_{n-1}+\cdots+a_{0}=0\right\}
\end{aligned}
$$
Show that $I$ is an ideal of $F[x]$ and find a generator for $I$.