English
If f and g are antitone maps, then x ↦ f(x) ∪ g(x) is antitone.
Русский
Если f и g антимонотонны, то x ↦ f(x) ∪ g(x) антимонотонна.
LaTeX
$$$ \operatorname{Antitone}(f) \land \operatorname{Antitone}(g) \Rightarrow \operatorname{Antitone}(x \mapsto f(x) \cup g(x)) $$$
Lean4
theorem union [Preorder β] {f g : β → Set α} (hf : Antitone f) (hg : Antitone g) : Antitone fun x => f x ∪ g x :=
hf.sup hg