∀ {G : Type u_2} {A : Type u_1} [inst : Group G] [inst_1 : MulAction G A] [inst_2 : MeasurableSpace A]
  [MeasurableConstSMul G A] (g : Gᵈᵐᵃ) (μ ν : MeasureTheory.Measure A),
  MeasureTheory.Measure.map (fun x => DomMulAct.mk.symm g⁻¹ • x) (μ + ν) =
    MeasureTheory.Measure.map (fun x => DomMulAct.mk.symm g⁻¹ • x) μ +
      MeasureTheory.Measure.map (fun x => DomMulAct.mk.symm g⁻¹ • x) ν