Let $X_0, X_1, X_2, \ldots$ each be a D16: Countable set such that
| (i) | $\bigcup_{n \in \mathbb{N}} X_n$ is the D77: Set union of $X_0, X_1, X_2, \ldots$ |
Then $\bigcup_{n \in \mathbb{N}} X_n$ is a D16: Countable set.