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
hJ : J ∈ biUnion π πi
⊢ J ∈ πi (biUnionIndex π πi J)
convert (π.mem_biUnion.1 hJ).choose_spec.2 <;> exact dif_pos hJ