-- Lean 4 proof step from Mathlib.CategoryTheory.Abelian.DiagramLemmas.Four (goal)
Goal:
C : Type u_1
inst : Category.{v_1, u_1} C
inst_1 : Abelian.{v_1, u_1} C
R₁ R₂ : ComposableArrows.{v_1, u_1} C 2
φ : R₁ ⟶ R₂
hR₁ : R₁.Exact
hR₂ : R₂.Exact
hR₁' : Epi (R₁.map' 1 2 mono_of_epi_of_mono_of_mono'._proof_6 mono_of_epi_of_epi_mono'._proof_1)
hR₂' : Epi (R₂.map' 1 2 mono_of_epi_of_mono_of_mono'._proof_6 mono_of_epi_of_epi_mono'._proof_1)
h₀ : Epi (app' φ 0 mono_of_epi_of_mono_of_mono'._proof_2)
h₁ : IsIso (app' φ 1 mono_of_epi_of_mono_of_mono'._proof_6)
ψ : mk₄ (X₀ := R₁.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ := R₁.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
    (X₃ := 0) (X₄ := 0)
    (R₁.map' 0 1 ⋯ ⋯ : R₁.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₁.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
    (R₁.map' 1 2 ⋯ ⋯ : R₁.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₁.obj (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
    0 0 ⟶
  mk₄ (X₀ := R₂.obj ⟨0, ⋯⟩) (X₁ := R₂.obj ⟨1, ⋯⟩) (X₃ := 0) (X₄ := 0)
    (R₂.map' 0 1 ⋯ ⋯ : R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj ⟨1, ⋯⟩)
    (R₂.map' 1 2 ⋯ ⋯ : R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj ⟨2, ⋯⟩) 0 0 :=
  homMk₄ (app' φ 0 ⋯) (app' φ 1 ⋯) (app' φ 2 ⋯) 0 0 ⋯ ⋯ ⋯ ⋯
refine_2 :
  (mk₂ (X₀ :=
      (mk₄ (X₀ := R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ := R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
              (X₃ := 0) (X₄ := 0)
              (R₂.map' 0 1 ⋯ ⋯ :
                R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
              (R₂.map' 1 2 ⋯ ⋯ :
                R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
              0 0).δ₀.obj
        (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 1 2 + 1)))
      (X₁ :=
      (mk₄ (X₀ := R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ := R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
              (X₃ := 0) (X₄ := 0)
              (R₂.map' 0 1 ⋯ ⋯ :
                R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
              (R₂.map' 1 2 ⋯ ⋯ :
                R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
              0 0).δ₀.obj
        (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 1 2 + 1)))
      (X₂ :=
      (mk₄ (X₀ := R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ := R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
              (X₃ := 0) (X₄ := 0)
              (R₂.map' 0 1 ⋯ ⋯ :
                R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
              (R₂.map' 1 2 ⋯ ⋯ :
                R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
              0 0).δ₀.obj
        (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 1 2 + 1)))
      ((mk₄ (X₀ := R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ :=
                R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₃ := 0) (X₄ := 0)
                (R₂.map' 0 1 ⋯ ⋯ :
                  R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                (R₂.map' 1 2 ⋯ ⋯ :
                  R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                0 0).δ₀.map'
          0 1 isComplex₂_iff._proof_2 ⋯ :
        (mk₄ (X₀ := R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ :=
                  R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₃ := 0) (X₄ := 0)
                  (R₂.map' 0 1 ⋯ ⋯ :
                    R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  (R₂.map' 1 2 ⋯ ⋯ :
                    R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  0 0).δ₀.obj
            (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 1 2 + 1)) ⟶
          (mk₄ (X₀ := R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ :=
                  R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₃ := 0) (X₄ := 0)
                  (R₂.map' 0 1 ⋯ ⋯ :
                    R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  (R₂.map' 1 2 ⋯ ⋯ :
                    R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  0 0).δ₀.obj
            (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 1 2 + 1)))
      ((mk₄ (X₀ := R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ :=
                R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₃ := 0) (X₄ := 0)
                (R₂.map' 0 1 ⋯ ⋯ :
                  R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                (R₂.map' 1 2 ⋯ ⋯ :
                  R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                0 0).δ₀.map'
          1 2 isComplex₂_iff._proof_4 ⋯ :
        (mk₄ (X₀ := R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ :=
                  R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₃ := 0) (X₄ := 0)
                  (R₂.map' 0 1 ⋯ ⋯ :
                    R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  (R₂.map' 1 2 ⋯ ⋯ :
                    R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  0 0).δ₀.obj
            (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 1 2 + 1)) ⟶
          (mk₄ (X₀ := R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ :=
                  R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₃ := 0) (X₄ := 0)
                  (R₂.map' 0 1 ⋯ ⋯ :
                    R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  (R₂.map' 1 2 ⋯ ⋯ :
                    R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  0 0).δ₀.obj
            (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 1 2 + 1)))).Exact
⊢ (mk₄ (X₀ := R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ := R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
      (X₃ := 0) (X₄ := 0)
      (R₂.map' 0 1 ⋯ ⋯ :
        R₂.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
      (R₂.map' 1 2 ⋯ ⋯ :
        R₂.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨2, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
      0 0).Exact
have refine_2₁ : (mk₄ (X₀ := R₂.obj (⟨0, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ := R₂.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
        (X₃ := 0) (X₄ := 0)
        (R₂.map' 0 1 sorry sorry :
          R₂.obj (⟨0, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
        (R₂.map' 1 2 sorry sorry :
          R₂.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₂.obj (⟨2, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
        0 0).δ₀.δ₀.Exact
Tactic: refine exact_of_δ₀ (hR₂.exact 0).exact_toComposableArrows (exact_of_δ₀ ?_ ?_)