∀ {ι : Type u_1} [inst : Fintype ι] (l : BoxIntegral.IntegrationParams) (I : BoxIntegral.Box ι) (c : NNReal)
  (π₀ : BoxIntegral.Prepartition I),
  l.toFilterDistortioniUnion I c π₀ = l.toFilterDistortion I c ⊓ Filter.principal {π | π.iUnion = π₀.iUnion}