refine isClosed_iff_clusterPt.2 (λ f hf => ?_)
U : Set ℂ
Q : Set ℂ → Set ℂ
ι : Type u_1
l : Filter ι
hU : IsOpen U
⊢ IsClosed (𝓗 U)
U : Set ℂ
Q : Set ℂ → Set ℂ
ι : Type u_1
l : Filter ι
hU : IsOpen U
f : 𝓒 U
hf : ClusterPt f (𝓟 (𝓗 U))
⊢ f ∈ 𝓗 U