Natural Number is Superset of its Union

Theorem
Let $n \in \N$ be a natural number as defined by the von Neumann construction.

Then:
 * $\bigcup n \subseteq n$

Proof
Let $n \in \N$.

From Natural Number is Transitive Set, $n$ is transitive.

From Class is Transitive iff Union is Subclass it follows directly that:
 * $\bigcup n \subseteq n$