∀ {α : Type u_1} {β : Type u_2} {mα : MeasurableSpace α} {mβ : MeasurableSpace β} {γ : Type u_4}
  {mγ : MeasurableSpace γ} (κ : ProbabilityTheory.Kernel (α × β) γ) (a : α), κ.sectR a = κ.comap (fun b => (a, b)) ⋯