no goals
α : Type u
β : Type v
ι : Sort w
a : α
s s₁ s₂ t : Set α
p p₁ p₂ : α → Prop
inst✝ : TopologicalSpace α
⊢ IsClosed s₁ → IsClosed s₂ → IsClosed (s₁ ∪ s₂)
simpa only [← isOpen_compl_iff, compl_union] using IsOpen.inter