/-- Suppose that a function `f : ℂ → E` is continuous on a closed rectangle with opposite corners at
`z w : ℂ`, is *real* differentiable at all but countably many points of the corresponding open
rectangle, and $\frac{\partial f}{\partial \bar z}$ is integrable on this rectangle. Then the
integral of `f` over the boundary of the rectangle is equal to the integral of
$2i\frac{\partial f}{\partial \bar z}=i\frac{\partial f}{\partial x}-\frac{\partial f}{\partial y}$
over the rectangle. -/
theorem integral_boundary_rect_of_hasFDerivAt_real_off_countable (f : ℂ → E) (f' : ℂ → ℂ →L[ℝ] E)
    (z w : ℂ) (s : Set ℂ) (hs : s.Countable)
    (Hc : ContinuousOn f ([[z.re, w.re]] ×ℂ [[z.im, w.im]]))
    (Hd : ∀ x ∈ Ioo (min z.re w.re) (max z.re w.re) ×ℂ Ioo (min z.im w.im) (max z.im w.im) \ s,
      HasFDerivAt f (f' x) x)
    (Hi : IntegrableOn (fun z => I • f' z 1 - f' z I) ([[z.re, w.re]] ×ℂ [[z.im, w.im]])) :
    (∫ x : ℝ in z.re..w.re, f (x + z.im * I)) - (∫ x : ℝ in z.re..w.re, f (x + w.im * I)) +
      I • (∫ y : ℝ in z.im..w.im, f (re w + y * I)) -
      I • ∫ y : ℝ in z.im..w.im, f (re z + y * I) =
      ∫ x : ℝ in z.re..w.re, ∫ y : ℝ in z.im..w.im, I • f' (x + y * I) 1 - f' (x + y * I) I := by
  set e : (ℝ × ℝ) ≃L[ℝ] ℂ := equivRealProdClm.symm
  have he : ∀ x y : ℝ, ↑x + ↑y * I = e (x, y) := fun x y => (mk_eq_add_mul_I x y).symm
  have he₁ : e (1, 0) = 1 := rfl; have he₂ : e (0, 1) = I := rfl
  simp only [he] at *
  set F : ℝ × ℝ → E := f ∘ e
  set F' : ℝ × ℝ → ℝ × ℝ →L[ℝ] E := fun p => (f' (e p)).comp (e : ℝ × ℝ →L[ℝ] ℂ)
  have hF' : ∀ p : ℝ × ℝ, (-(I • F' p)) (1, 0) + F' p (0, 1) = -(I • f' (e p) 1 - f' (e p) I) := by
    rintro ⟨x, y⟩
    simp only [ContinuousLinearMap.neg_apply, ContinuousLinearMap.smul_apply,
      ContinuousLinearMap.comp_apply, ContinuousLinearEquiv.coe_coe, he₁, he₂, neg_add_eq_sub,
      neg_sub]
  set R : Set (ℝ × ℝ) := [[z.re, w.re]] ×ˢ [[w.im, z.im]]
  set t : Set (ℝ × ℝ) := e ⁻¹' s
  rw [uIcc_comm z.im] at Hc Hi; rw [min_comm z.im, max_comm z.im] at Hd
  have hR : e ⁻¹' ([[z.re, w.re]] ×ℂ [[w.im, z.im]]) = R := rfl
  have htc : ContinuousOn F R := Hc.comp e.continuousOn hR.ge
  have htd :
    ∀ p ∈ Ioo (min z.re w.re) (max z.re w.re) ×ˢ Ioo (min w.im z.im) (max w.im z.im) \ t,
      HasFDerivAt F (F' p) p :=
    fun p hp => (Hd (e p) hp).comp p e.hasFDerivAt
  simp_rw [← intervalIntegral.integral_smul, intervalIntegral.integral_symm w.im z.im, ←
    intervalIntegral.integral_neg, ← hF']
  refine' (integral2_divergence_prod_of_hasFDerivWithinAt_off_countable (fun p => -(I • F p)) F
    (fun p => -(I • F' p)) F' z.re w.im w.re z.im t (hs.preimage e.injective)
    (htc.const_smul _).neg htc (fun p hp => ((htd p hp).const_smul I).neg) htd _).symm
  rw [← (volume_preserving_equiv_real_prod.symm _).integrableOn_comp_preimage
    (MeasurableEquiv.measurableEmbedding _)] at Hi
  

Proof state:
E : Type u
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℂ E
inst✝ : CompleteSpace E
f : ℂ → E
f' : ℂ → ℂ →L[ℝ] E
z w : ℂ
s : Set ℂ
hs : Set.Countable s
Hc : ContinuousOn f ([[z.re, w.re]] ×ℂ [[w.im, z.im]])
Hd : ∀ x ∈ Ioo (min z.re w.re) (max z.re w.re) ×ℂ Ioo (min w.im z.im) (max w.im z.im) \ s, HasFDerivAt f (f' x) x
Hi :
  IntegrableOn ((fun z => I • (f' z) 1 - (f' z) I) ∘ ⇑measurableEquivRealProd.symm)
    (⇑measurableEquivRealProd.symm ⁻¹' ([[z.re, w.re]] ×ℂ [[w.im, z.im]]))
e : (ℝ × ℝ) ≃L[ℝ] ℂ := ContinuousLinearEquiv.symm equivRealProdClm
he₁ : (ContinuousLinearEquiv.symm equivRealProdClm) (1, 0) = 1
he₂ : (ContinuousLinearEquiv.symm equivRealProdClm) (0, 1) = I
he : ℝ → ℝ → True
F : ℝ × ℝ → E := f ∘ ⇑e
F' : ℝ × ℝ → ℝ × ℝ →L[ℝ] E := fun p => ContinuousLinearMap.comp (f' (e p)) ↑e
hF' : ∀ (p : ℝ × ℝ), (-(I • F' p)) (1, 0) + (F' p) (0, 1) = -(I • (f' (e p)) 1 - (f' (e p)) I)
R : Set (ℝ × ℝ) := [[z.re, w.re]] ×ˢ [[w.im, z.im]]
t : Set (ℝ × ℝ) := ⇑e ⁻¹' s
hR : ⇑e ⁻¹' ([[z.re, w.re]] ×ℂ [[w.im, z.im]]) = R
htc : ContinuousOn F R
htd : ∀ p ∈ Ioo (min z.re w.re) (max z.re w.re) ×ˢ Ioo (min w.im z.im) (max w.im z.im) \ t, HasFDerivAt F (F' p) p
⊢ IntegrableOn (fun x => ((fun p => -(I • F' p)) x) (1, 0) + (F' x) (0, 1)) ([[z.re, w.re]] ×ˢ [[w.im, z.im]])

Next tactic:
simpa only [hF'] using Hi.neg