Let $T = (X, \mathcal{T})$ be a D1106: Topological space.
A D11: Set $E \subseteq X$ is clopen in $T$ if and only if

(1)	$E \in \mathcal{T}$ (D97: Open set)
(2)	$X \setminus E \in \mathcal{T}$ (D98: Closed set)