∀ {α : Type u_1} {m : MeasurableSpace α} (s t : MeasureTheory.SignedMeasure α) (x : ℕ → Set α),
  (∀ (i : ℕ), MeasurableSet (x i)) →
    Pairwise (Function.onFun Disjoint x) →
      HasSum (fun i => { re := ↑s (x i), im := ↑t (x i) }) { re := ↑s (⋃ i, x i), im := ↑t (⋃ i, x i) }