Set Union Preserves Subsets/Proof 2

Theorem
Let $A, B, S, T$ be sets.

Then:
 * $A \subseteq B, \ S \subseteq T \implies A \cup S \subseteq B \cup T$

Proof
By Subset Relation is Transitive, $\subseteq$ is a transitive relation.

By Subset Relation is Compatible with Union Operation (Proof 2), $\subset$ is compatible with $\cup$.

Thus the theorem holds by Operating on Transitive Relationships Compatible with Operation.