∀ {X : Type u_1} {𝓑 𝓧 : MeasurableSpace X} {π : ProbabilityTheory.Kernel X X} (h𝓑𝓧 : 𝓑 ≤ 𝓧),
  π.IsProper ↔ ∀ ⦃B : Set X⦄ (hB : MeasurableSet B) (x : X), (π.restrict ⋯) x = B.indicator (fun x => 1) x • π x