Let $X$ be a D11: Set such that
(i) | $E \subseteq X$ is a D78: Subset of $X$ |
(ii) | $X \setminus E$ is the D79: Set complement of $E$ within $X$ |
(iii) | $I_{X \setminus E} : X \to \{ 0, 1 \}$ is an D41: Indicator function on $X$ with respect to $X \setminus E$ |
Then
\begin{equation}
I_{X \setminus E}
= 1 - I_E
\end{equation}