no goals
ι : Type u_1
I J J₁ J₂ : Box ι
π π₁ π₂ : Prepartition I
x : ι → ℝ
πi πi₁ πi₂ : (J : Box ι) → Prepartition J
⊢ Prepartition.iUnion (restrict π J) = ↑J ∩ Prepartition.iUnion π
simp [restrict, ← inter_iUnion, ← iUnion_def]