/-- The symmetric difference of two Hahn decompositions has measure zero. -/
theorem of_symmDiff_compl_positive_negative {s : SignedMeasure α} {i j : Set α}
    (hi : MeasurableSet i) (hj : MeasurableSet j) (hi' : 0 ≤[i] s ∧ s ≤[iᶜ] 0)
    (hj' : 0 ≤[j] s ∧ s ≤[jᶜ] 0) : s (i ∆ j) = 0 ∧ s (iᶜ ∆ jᶜ) = 0 := by
  rw [restrict_le_restrict_iff s 0, restrict_le_restrict_iff 0 s] at hi' hj'
  constructor
  · rw [Set.symmDiff_def, Set.diff_eq_compl_inter, Set.diff_eq_compl_inter, of_union,
      le_antisymm (hi'.2 (hi.compl.inter hj) (Set.inter_subset_left _ _))
        (hj'.1 (hi.compl.inter hj) (Set.inter_subset_right _ _)),
      le_antisymm (hj'.2 (hj.compl.inter hi) (Set.inter_subset_left _ _))
        (hi'.1 (hj.compl.inter hi) (Set.inter_subset_right _ _)),
      zero_apply, zero_apply, zero_add]
    · exact
        Set.disjoint_of_subset_left (Set.inter_subset_left _ _)
          (Set.disjoint_of_subset_right (Set.inter_subset_right _ _)
            (disjoint_comm.1 (IsCompl.disjoint isCompl_compl)))
    · exact hj.compl.inter hi
    · exact hi.compl.inter hj
  · rw [Set.symmDiff_def, Set.diff_eq_compl_inter, Set.diff_eq_compl_inter, compl_compl,
      compl_compl, of_union,
      le_antisymm (hi'.2 (hj.inter hi.compl) (Set.inter_subset_right _ _))
        (hj'.1 (hj.inter hi.compl) (Set.inter_subset_left _ _)),
      le_antisymm (hj'.2 (hi.inter hj.compl) (Set.inter_subset_right _ _))
        (hi'.1 (hi.inter hj.compl) (Set.inter_subset_left _ _)),
      zero_apply, zero_apply, zero_add]
    · exact
        Set.disjoint_of_subset_left (Set.inter_subset_left _ _)
          (Set.disjoint_of_subset_right (Set.inter_subset_right _ _)
            (IsCompl.disjoint isCompl_compl))
    · exact hj.inter hi.compl
    · exact hi.inter hj.compl
  

Proof state:
α : Type u_1
β : Type u_2
inst✝³ : MeasurableSpace α
M : Type u_3
inst✝² : AddCommMonoid M
inst✝¹ : TopologicalSpace M
inst✝ : OrderedAddCommMonoid M
s✝ : SignedMeasure α
i✝ j✝ : Set α
s : SignedMeasure α
i j : Set α
hi : MeasurableSet i
hj : MeasurableSet j
hi' : (∀ ⦃j : Set α⦄, MeasurableSet j → j ⊆ i → ↑0 j ≤ ↑s j) ∧ ∀ ⦃j : Set α⦄, MeasurableSet j → j ⊆ iᶜ → ↑s j ≤ ↑0 j
hj' : restrict 0 j ≤ restrict s j ∧ ∀ ⦃j_1 : Set α⦄, MeasurableSet j_1 → j_1 ⊆ jᶜ → ↑s j_1 ≤ ↑0 j_1
⊢ MeasurableSet j
α : Type u_1
β : Type u_2
inst✝³ : MeasurableSpace α
M : Type u_3
inst✝² : AddCommMonoid M
inst✝¹ : TopologicalSpace M
inst✝ : OrderedAddCommMonoid M
s✝ : SignedMeasure α
i✝ j✝ : Set α
s : SignedMeasure α
i j : Set α
hi : MeasurableSet i
hj : MeasurableSet j
hi' : restrict 0 i ≤ restrict s i ∧ ∀ ⦃j : Set α⦄, MeasurableSet j → j ⊆ iᶜ → ↑s j ≤ ↑0 j
hj' : restrict 0 j ≤ restrict s j ∧ ∀ ⦃j_1 : Set α⦄, MeasurableSet j_1 → j_1 ⊆ jᶜ → ↑s j_1 ≤ ↑0 j_1
⊢ MeasurableSet i
α : Type u_1
β : Type u_2
inst✝³ : MeasurableSpace α
M : Type u_3
inst✝² : AddCommMonoid M
inst✝¹ : TopologicalSpace M
inst✝ : OrderedAddCommMonoid M
s✝ : SignedMeasure α
i✝ j✝ : Set α
s : SignedMeasure α
i j : Set α
hi : MeasurableSet i
hj : MeasurableSet j
hi' : restrict 0 i ≤ restrict s i ∧ ∀ ⦃j : Set α⦄, MeasurableSet j → j ⊆ iᶜ → ↑s j ≤ ↑0 j
hj' : restrict 0 j ≤ restrict s j ∧ restrict s jᶜ ≤ restrict 0 jᶜ
⊢ MeasurableSet jᶜ
α : Type u_1
β : Type u_2
inst✝³ : MeasurableSpace α
M : Type u_3
inst✝² : AddCommMonoid M
inst✝¹ : TopologicalSpace M
inst✝ : OrderedAddCommMonoid M
s✝ : SignedMeasure α
i✝ j✝ : Set α
s : SignedMeasure α
i j : Set α
hi : MeasurableSet i
hj : MeasurableSet j
hi' : restrict 0 i ≤ restrict s i ∧ restrict s iᶜ ≤ restrict 0 iᶜ
hj' : restrict 0 j ≤ restrict s j ∧ restrict s jᶜ ≤ restrict 0 jᶜ
⊢ MeasurableSet iᶜ

Next tactic:
all_goals measurability