Let $J$ be a D16: Countable set such that
| (i) | $X_j$ is a D16: Countable set for each $j \in J$ |
| (ii) | $\bigcup_{j \in J} X_j$ is the D77: Set union of $\{ X_j \}_{j \in J}$ |
Then $\bigcup_{j \in J} X_j$ is a D16: Countable set.