∀ {α : Type u_1} {C : Set (Set α)} {s : Set α} {I : Finset (Set α)} {G : Type u_2} [inst : AddCommMonoid G]
  {m : MeasureTheory.AddContent G C} (hC : MeasureTheory.IsSetSemiring C) (hs : s ∈ C) (hI : ↑I ⊆ C),
  (∀ t ∈ I, t ⊆ s) → (↑I).PairwiseDisjoint id → m s = ∑ i ∈ I, m i + ∑ i ∈ hC.disjointOfDiffUnion hs hI, m i