∀ {α : Type u_1} {m : MeasurableSpace α} {μ : MeasureTheory.Measure α} {E : Type u_2} [inst : NormedAddCommGroup E]
  [inst_1 : NormedSpace ℝ E] {f g : α → E},
  MeasureTheory.Integrable f μ →
    MeasureTheory.Integrable g μ → μ.withDensityᵥ (f + g) = μ.withDensityᵥ f + μ.withDensityᵥ g