Solution
= Solution
Every subset is contained in its <closure>, giving $\overline A\subseteq\overline{\overline A}$. On the other hand, $\overline A$ is already <closed> by its definition as an <intersection> of <closed> sets. It is therefore one of the <closed> supersets used when taking its own <closure>, so $\overline{\overline A}\subseteq\overline A$. Hence
$$
\boxed{\overline{\overline A}=\overline A.}
$$
This is the <idempotence> of the <closure> operation.