-- Lean 4 proof step from Mathlib.Probability.Kernel.RadonNikodym (goal)
Goal:
α : Type u_1
γ : Type u_2
mα : MeasurableSpace α
mγ : MeasurableSpace γ
hαγ : MeasurableSpace.CountableOrCountablyGenerated α γ
κ η : Kernel α γ
inst : IsFiniteKernel κ
inst_1 : IsFiniteKernel η
a : α
s : Set γ
hs : MeasurableSet s
hm : MeasurableSet (κ.mutuallySingularSetSlice η a)
h :
  HAdd.hAdd (α := ℝ≥0∞) (β := ℝ≥0∞)
        ((((η.withDensity (κ.rnDeriv η) : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞)
            (s ∩ κ.mutuallySingularSetSlice η a) :
          ℝ≥0∞)
        ((((η.withDensity (κ.rnDeriv η) : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞)
            (s \ κ.mutuallySingularSetSlice η a) :
          ℝ≥0∞) +
      HAdd.hAdd (α := ℝ≥0∞) (β := ℝ≥0∞)
        ((((κ : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞) (s ∩ κ.mutuallySingularSetSlice η a) : ℝ≥0∞)
        ((((κ.singularPart η : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞)
            (s \ κ.mutuallySingularSetSlice η a) :
          ℝ≥0∞) =
    HAdd.hAdd (α := ℝ≥0∞) (β := ℝ≥0∞)
      ((((κ : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞) (s ∩ κ.mutuallySingularSetSlice η a) : ℝ≥0∞)
      ((((κ : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞) (s \ κ.mutuallySingularSetSlice η a) : ℝ≥0∞)
h₁ :
  HAdd.hAdd (α := ℝ≥0∞) (β := ℝ≥0∞)
        ((((η.withDensity (κ.rnDeriv η) : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞)
            (s ∩ κ.mutuallySingularSetSlice η a) :
          ℝ≥0∞)
        ((((η.withDensity (κ.rnDeriv η) : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞)
            (s \ κ.mutuallySingularSetSlice η a) :
          ℝ≥0∞) +
      HAdd.hAdd (α := ℝ≥0∞) (β := ℝ≥0∞)
        ((((κ : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞) (s ∩ κ.mutuallySingularSetSlice η a) : ℝ≥0∞)
        (0 : ℝ≥0∞) =
    HAdd.hAdd (α := ℝ≥0∞) (β := ℝ≥0∞)
      ((((κ : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞) (s ∩ κ.mutuallySingularSetSlice η a) : ℝ≥0∞)
      ((((κ : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞) (s \ κ.mutuallySingularSetSlice η a) : ℝ≥0∞)
⊢ HAdd.hAdd (α := ℝ≥0∞) (β := ℝ≥0∞)
        ((((η.withDensity (κ.rnDeriv η) : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞)
            (s ∩ κ.mutuallySingularSetSlice η a) :
          ℝ≥0∞)
        ((((η.withDensity (κ.rnDeriv η) : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞)
            (s \ κ.mutuallySingularSetSlice η a) :
          ℝ≥0∞) +
      HAdd.hAdd (α := ℝ≥0∞) (β := ℝ≥0∞)
        ((((κ.singularPart η : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞)
            (s ∩ κ.mutuallySingularSetSlice η a) :
          ℝ≥0∞)
        ((((κ.singularPart η : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞)
            (s \ κ.mutuallySingularSetSlice η a) :
          ℝ≥0∞) =
    HAdd.hAdd (α := ℝ≥0∞) (β := ℝ≥0∞)
      ((((κ : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞) (s ∩ κ.mutuallySingularSetSlice η a) : ℝ≥0∞)
      ((((κ : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞) (s \ κ.mutuallySingularSetSlice η a) : ℝ≥0∞)
have h₂ : HAdd.hAdd (β := ℝ≥0∞)
    (HAdd.hAdd (α := ℝ≥0∞) (β := ℝ≥0∞)
      ((((η.withDensity (κ.rnDeriv η) : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞)
          (s ∩ κ.mutuallySingularSetSlice η a) :
        ℝ≥0∞)
      ((((η.withDensity (κ.rnDeriv η) : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞)
          (s \ κ.mutuallySingularSetSlice η a) :
        ℝ≥0∞))
    ((((κ : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞) (s ∩ κ.mutuallySingularSetSlice η a) : ℝ≥0∞) =
  HAdd.hAdd (α := ℝ≥0∞) (β := ℝ≥0∞)
    ((((κ : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞) (s ∩ κ.mutuallySingularSetSlice η a) : ℝ≥0∞)
    ((((κ : (a : α) → Measure γ) a : Measure γ) : (a : Set γ) → ℝ≥0∞) (s \ κ.mutuallySingularSetSlice η a) : ℝ≥0∞)
Tactic: rw [singularPart_of_subset_mutuallySingularSetSlice (hs.inter hm) inter_subset_right,
    singularPart_of_subset_compl_mutuallySingularSetSlice (diff_subset_iff.mpr (by simp)),
    add_zero, withDensity_rnDeriv_of_subset_mutuallySingularSetSlice inter_subset_right,
    zero_add, withDensity_rnDeriv_of_subset_compl_mutuallySingularSetSlice (hs.diff hm)
      (diff_subset_iff.mpr (by simp)), add_comm]