∀ {α : Type u_1} {β : Type u_2} {m : MeasurableSpace α} {μ : MeasureTheory.Measure α} {x : β} {t : Set β} {s : Set α}
  [inst : Decidable (x ∈ t)], μ (if x ∈ t then s else ∅) = t.indicator (fun x => μ s) x