Union of Union of Relation is Union of Domain with Image

Theorem
Let $V$ be a basic universe.

Let $\RR \subseteq V \times V$ be a relation.

Let $\Dom \RR$ denote the domain of $\RR$.

Then:
 * $\map \bigcup {\bigcup \RR} = \Dom \RR \cup \Img \RR$

where:
 * $\bigcup \RR$ denotes the union of $\RR$
 * $\Dom \RR$ denotes the domain of $\RR$
 * $\Img \RR$ denotes the image of $\RR$.