A D4: Binary relation $(X \times Y, R)$ is right-unique if and only if
\begin{equation}
\forall \, x \in X : \forall \, y, y' \in Y
\left( (x, y), (x, y') \in R \quad \implies \quad y = y' \right)
\end{equation}
A D4: Binary relation $(X \times Y, R)$ is right-unique if and only if
\begin{equation}
\forall \, x \in X : \forall \, y, y' \in Y
\left( x R y, x R y' \quad \implies \quad y = y' \right)
\end{equation}
