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
h : Prepartition.iUnion π₂ = ↑I \ Prepartition.iUnion π₁
⊢ Prepartition.iUnion π₁ ∪ Prepartition.iUnion π₂ = ↑I
simp [h, π₁.iUnion_subset]