Let $\displaystyle (E_k)_{k=1}^{\infty}$ be a sequence of measurable sets in $\displaystyle \mathbb{R}^n$. Let $\displaystyle E$ be a subset of $\displaystyle \mathbb{R}^n$ so that $\displaystyle x \in E$ if and only if $\displaystyle x$ belongs to exactly 2 of the sets $\displaystyle E_k$. Determine whether E is measurable. If it is, show it. If not, give a counterexample.

I suppose it is measurable? No? Can't think of any counterexample.

Thanks in advance.