∀ {α : Type u_1} {m : MeasurableSpace α} {μ ν ξ : MeasureTheory.Measure α} [μ.HaveLebesgueDecomposition ν],
  ξ.AbsolutelyContinuous μ → ξ.AbsolutelyContinuous ν → ξ.AbsolutelyContinuous (ν.withDensity (μ.rnDeriv ν))