Union of Subsets is Subset

Theorem
Let $S_1$, $S_2$, and $T$ be sets.

Let $S_1$ and $S_2$ both be subsets of $T$.

Then:
 * $S_1 \cup S_2 \subseteq T$

That is:
 * $\paren {S_1 \subseteq T} \land \paren {S_2 \subseteq T} \implies \paren {S_1 \cup S_2} \subseteq T$