Set Union is Self-Distributive/Sets of Sets

Theorem
Let $A$ and $B$ denote sets of sets.

Then:
 * $\displaystyle \bigcup \left({A \cup B}\right) = \left({\bigcup A}\right) \cup \left({\bigcup B}\right)$

where $\displaystyle \bigcup A$ denotes the union of $A$.