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