∀ {ι : Type u_1} {I : BoxIntegral.Box ι} (π₁ π₂ : BoxIntegral.Prepartition I) (h : Disjoint π₁.iUnion π₂.iUnion),
  π₁.disjUnion π₂ h = { boxes := π₁.boxes ∪ π₂.boxes, le_of_mem' := ⋯, pairwiseDisjoint := ⋯ }