∀ {α : Type u_1} {β : Type u_2} {γ : Type u_3} {mα : MeasurableSpace α} {mβ : MeasurableSpace β}
  {mγ : MeasurableSpace γ} (κ η : ProbabilityTheory.Kernel α β),
  ProbabilityTheory.Kernel.prodMkLeft γ (κ + η) =
    ProbabilityTheory.Kernel.prodMkLeft γ κ + ProbabilityTheory.Kernel.prodMkLeft γ η