Intersection of All Subrings Containing Subset is Smallest

Theorem
Let $\struct {R, +, \circ}$ be a ring.

Let $S \subseteq R$ be a subset of $R$.

Let $L$ be the intersection of the set of all subrings of $R$ containing $S$.

Then $L$ is the smallest subring of $R$ containing $S$.

Proof
From Intersection of Subrings is Subring, $L$ is indeed a subring of $R$.

Let $T$ be a subring of $R$ containing $S$.

Let $x, y \in L$.

By the Subring Test, we have that:
 * $x - y \in L$
 * $x \circ y \in L$

By Intersection is Largest Subset, it follows that $x, y \in T$.

But $T$ is also a subring of $R$.

So, by the Subring Test again, we have that:
 * $x - y \in T$
 * $x \circ y \in T$

So by definition of subset, $L \subseteq T$.