Let $f : X \to Y$ be a D1519: Constant map from $X$ to $Y$ such that
(i) | $B \subseteq Y$ is a D78: Subset of $Y$ |
Then
\begin{equation}
f^{-1}(B)
\in \{ \emptyset, X \}
\end{equation}
(i) | $B \subseteq Y$ is a D78: Subset of $Y$ |