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- If $S \subset T$, then $\bigcup S \in T$. This includes the "empty union", i.e., if $S = \emptyset$, then $\bigcup S = \emptyset \in T$.
- If $S \subset T$, and $S$ is finite, then $\bigcap S \in T$. This includes the "empty intersection", i.e., if $S = \emptyset$, then we define $\bigcap S = X$, and so $X \in T$.
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