Closed Sets of Fortissimo Space

Theorem
Let $T = \struct {S, \tau_p}$ be a Fortissimo space.

Then $H \subseteq S$ is closed in $T$ :
 * $p \in H$

or
 * $H$ is countable

or both.

Proof
By definition of a Fortissimo space, $U \subseteq S$ is open in $T$ :
 * $p \in \relcomp S U$

or
 * $\relcomp S U$ is countable

or both.

The result follows from the definition of closed set.