-- 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
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)
h₀ : Epi (app' φ 0 mono_of_epi_of_mono_of_mono'._proof_2)
h₁ : Epi (app' φ 2 mono_of_epi_of_epi_mono'._proof_1)
ψ : mk₃ (X₀ := R₁.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ := R₁.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
    (X₃ := R₁.obj' 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 ⟶
  mk₃ (X₀ := R₂.obj ⟨0, ⋯⟩) (X₁ := R₂.obj ⟨1, ⋯⟩) (X₃ := R₁.obj' 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 :=
  homMk₃ (app' φ 0 ⋯) (app' φ 1 ⋯) (app' φ 2 ⋯)
    (𝟙
      ((mk₃ (X₀ := R₁.obj (⟨0, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ :=
            R₁.obj (⟨1, ⋯⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₃ := R₁.obj' 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).obj'
        3 _proof_300✝))
    ⋯ ⋯ ⋯
⊢ Epi (app' φ 1 mono_of_epi_of_mono_of_mono'._proof_6)
have refine : {
    X₁ :=
      (mk₂
            ((mk₃ (R₁.map' 0 1 sorry sorry) (R₁.map' 1 2 sorry sorry) 0).map' 1 2 mono_of_epi_of_mono_of_mono'._proof_6
              mono_of_epi_of_mono_of_mono'._proof_4)
            ((mk₃ (R₁.map' 0 1 sorry sorry) (R₁.map' 1 2 sorry sorry) 0).map' 2 3 mono_of_epi_of_mono_of_mono'._proof_4
              mono_of_epi_of_mono_of_mono'._proof_7)).obj
        ⟨0, sorry⟩,
    X₂ :=
      (mk₂
            ((mk₃ (R₁.map' 0 1 sorry sorry) (R₁.map' 1 2 sorry sorry) 0).map' 1 2 mono_of_epi_of_mono_of_mono'._proof_6
              mono_of_epi_of_mono_of_mono'._proof_4)
            ((mk₃ (R₁.map' 0 1 sorry sorry) (R₁.map' 1 2 sorry sorry) 0).map' 2 3 mono_of_epi_of_mono_of_mono'._proof_4
              mono_of_epi_of_mono_of_mono'._proof_7)).obj
        ⟨1, sorry⟩,
    X₃ :=
      (mk₂
            ((mk₃ (R₁.map' 0 1 sorry sorry) (R₁.map' 1 2 sorry sorry) 0).map' 1 2 mono_of_epi_of_mono_of_mono'._proof_6
              mono_of_epi_of_mono_of_mono'._proof_4)
            ((mk₃ (R₁.map' 0 1 sorry sorry) (R₁.map' 1 2 sorry sorry) 0).map' 2 3 mono_of_epi_of_mono_of_mono'._proof_4
              mono_of_epi_of_mono_of_mono'._proof_7)).obj
        ⟨2, sorry⟩,
    f :=
      (mk₂
            ((mk₃ (X₀ := R₁.obj (⟨0, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ :=
                  R₁.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₃ := R₁.obj' 0 sorry)
                  (R₁.map' 0 1 sorry sorry :
                    R₁.obj (⟨0, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₁.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  (R₁.map' 1 2 sorry sorry :
                    R₁.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₁.obj (⟨2, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  0).map'
              1 2 mono_of_epi_of_mono_of_mono'._proof_6 mono_of_epi_of_mono_of_mono'._proof_4)
            ((mk₃ (X₀ := R₁.obj (⟨0, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ :=
                  R₁.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₃ := R₁.obj' 0 sorry)
                  (R₁.map' 0 1 sorry sorry :
                    R₁.obj (⟨0, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₁.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  (R₁.map' 1 2 sorry sorry :
                    R₁.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₁.obj (⟨2, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  0).map'
              2 3 mono_of_epi_of_mono_of_mono'._proof_4 mono_of_epi_of_mono_of_mono'._proof_7)).map'
        0 1 isComplex₂_iff._proof_2 isComplex₂_iff._proof_4,
    g :=
      (mk₂
            ((mk₃ (X₀ := R₁.obj (⟨0, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ :=
                  R₁.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₃ := R₁.obj' 0 sorry)
                  (R₁.map' 0 1 sorry sorry :
                    R₁.obj (⟨0, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₁.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  (R₁.map' 1 2 sorry sorry :
                    R₁.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₁.obj (⟨2, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  0).map'
              1 2 mono_of_epi_of_mono_of_mono'._proof_6 mono_of_epi_of_mono_of_mono'._proof_4)
            ((mk₃ (X₀ := R₁.obj (⟨0, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₁ :=
                  R₁.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1))) (X₃ := R₁.obj' 0 sorry)
                  (R₁.map' 0 1 sorry sorry :
                    R₁.obj (⟨0, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₁.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  (R₁.map' 1 2 sorry sorry :
                    R₁.obj (⟨1, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)) ⟶ R₁.obj (⟨2, sorry⟩ : Fin (HAdd.hAdd (α := ℕ) 2 1)))
                  0).map'
              2 3 mono_of_epi_of_mono_of_mono'._proof_4 mono_of_epi_of_mono_of_mono'._proof_7)).map'
        1 2 isComplex₂_iff._proof_4 isComplex₂_iff._proof_5,
    zero := sorry : ShortComplex.{v_1, u_1} C }.Exact
Tactic: refine epi_of_epi_of_epi_of_mono' ψ (exact₂_mk _ (by simp) ?_)
    (hR₂.exact 0).exact_toComposableArrows (by simp)
    h₀ h₁ (by dsimp [ψ]; infer_instance)