A D4: Binary relation $M = (X \times Y, f)$ is a map if and only if

(1)	$\forall \, x \in X : \forall \, y, y' \in Y \, ((x, y), (x, y') \in f \quad \Rightarrow \quad y = y')$ (D358: Right-unique binary relation)
(2)	$\forall \, x \in X : \exists \, y \in Y : (x, y) \in f$ (D359: Left-total binary relation)