Let $f : X \to Y$ be a D18: Map such that
| (i) | $B_n \subseteq Y$ is a D78: Subset of $Y$ for each $n \in \mathbb{N}$ |
Then
\begin{equation}
f^{-1} \left( \bigcup_{n \in \mathbb{N}} B_n \right)
= \bigcup_{n \in \mathbb{N}} f^{-1}(B_n)
\end{equation}