Subset of Discrete Set of Subsets is Discrete

Theorem
Let $T = \struct {S, \tau}$ be a topological space.

Let $\FF \subseteq \powerset S$ be a set of subsets of $S$.

Let $\GG \subseteq \FF$.

If $\FF$ is discrete then $\GG$ is discrete.

Proof
We prove the contrapositive statement:
 * If $\GG$ is not locally finite then $\FF$ is not locally finite.

Let $\GG$ not be discrete.

By definition of discrete:
 * $\exists x \in S : \forall N \subseteq S : N$ is a neighborhood of $x : \exists X_1, X_2 \in \GG : X_1 \ne X_2: X_1 \cap N \ne \O, X_2 \cap N \ne \O$

By definition of subset:
 * $\forall X \in \GG : X \in \FF$

Hence:
 * $\exists x \in S : \forall N \subseteq S : N$ is a neighborhood of $x : \exists X_1, X_2 \in \FF : X_1 \ne X_2: X_1 \cap N \ne \O, X_2 \cap N \ne \O$

By definition, $\FF$ is not discrete.

The result follows from Rule of Transposition.