Let $f : X \to \mathbb{R}$ be a D4364: Real function.
Let $a, b \in \mathbb{R}$ each be a D993: Real number such that

(i)	$a \leq b$

Then \begin{equation} \{ x \in X : f(x) \leq a \} \subseteq \{ x \in X : f(x) \leq b \} \end{equation}