# Complement of Subset with Property (S) is Closed under Directed Suprema

## Theorem

Let $L = \struct {S, \preceq}$ be an up-complete ordered set.

Let $X$ be a subset of $S$ with property (S).

Then $\relcomp S X$ is closed under directed suprema.

## Proof

Let $D$ be a directed subset of $S$ such that:

$D \subseteq \relcomp S X$

Aiming for a contradiction, suppose

$\sup D \notin \relcomp S X$

By definition of relative complement:

$\sup D \in X$

By definition of property (S):

$\exists y \in D: \forall x \in D: y \preceq x \implies x \in X$

By definition of reflexivity:

$y \in X$

By definitions of intersection and non-empty:

$D \cap X \ne \O$
$D \subseteq \relcomp S X$

$\blacksquare$