Image of Union under Relation/General Result

Theorem
Let $\mathcal R \subseteq S \times T$ be a relation.

Let $\mathcal P \left({S}\right)$ be the power set of $S$.

Let $\mathbb S \subseteq \mathcal P \left({S}\right)$.

Then:
 * $\displaystyle \mathcal R \left({\bigcup \mathbb S}\right) = \bigcup_{X \mathop \in \mathbb S} \mathcal R \left({X}\right)$