refine isClosed_biInter (λ z hz => isClosed_ball.preimage ?_)
U : Set ℂ
Q : Set ℂ → Set ℂ
ι : Type u_1
l : Filter ι
hU : IsOpen U
⊢ IsClosed (⋂ i ∈ U, {x | x i ∈ closedBall 0 1})
U : Set ℂ
Q : Set ℂ → Set ℂ
ι : Type u_1
l : Filter ι
hU : IsOpen U
z : ℂ
hz : z ∈ U
⊢ Continuous fun x => x z