Let $X$ and $Y$ each be a D11: Set such that

(i)	$$\begin{equation} \{ \{ x \} : x \in X \} \subseteq Y \end{equation}$$

The singleton map from $X$ to $Y$ is the D18: Map $$\begin{equation} X \to Y, \quad x \mapsto \{ x \} \end{equation}$$