-- Lean 4 proof step from Mathlib.Algebra.Homology.BifunctorShift (goal)
Goal:
C₁ : Type u_1
C₂ : Type u_2
D : Type u_3
inst : Category.{v_1, u_1} C₁
inst_1 : Category.{v_2, u_2} C₂
inst_2 : Category.{v_3, u_3} D
inst_3 : Preadditive.{v_1, u_1} C₁
inst_4 : HasZeroMorphisms.{v_2, u_2} C₂
inst_5 : Preadditive.{v_3, u_3} D
K₁ L₁ : CochainComplex.{v_1, u_1, 0} C₁ ℤ
f₁ : K₁ ⟶ L₁
K₂ : CochainComplex.{v_2, u_2, 0} C₂ ℤ
F : C₁ ⥤ CategoryTheory.Functor.{v_2, v_3, u_2, u_3} C₂ D
inst_6 : F.Additive
inst_7 : ∀ (X₁ : C₁), (F.obj X₁).PreservesZeroMorphisms
x : ℤ
inst_8 : K₁.HasMapBifunctor K₂ F
i j : ℤ
h : (ComplexShape.up ℤ).Rel i j
⊢ Eq (α :=
    Quiver.Hom (V := HomologicalComplex.{v_3, u_3, 0} D (ComplexShape.up ℤ))
      (X
        (Functor.obj
          (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
            (Functor.obj (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
          K₂)
        i)
      (X
        (Functor.obj (HomologicalComplex₂.shiftFunctor₁ D x)
          (Functor.obj (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ)) K₁) K₂))
        j))
    (Iso.hom
        ((fun (x_1 : ℤ) ↦
            (Iso.refl
                (X
                  (Functor.obj
                    (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
                      (Functor.obj
                        (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
                    K₂)
                  x_1) :
              Iso (C := HomologicalComplex.{v_3, u_3, 0} D (ComplexShape.up ℤ))
                (X
                  (Functor.obj
                    (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
                      (Functor.obj
                        (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
                    K₂)
                  x_1)
                (X
                  (Functor.obj
                    (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
                      (Functor.obj
                        (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
                    K₂)
                  x_1)))
          i) ≫
      d
        (Functor.obj (HomologicalComplex₂.shiftFunctor₁ D x)
          (Functor.obj (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ)) K₁) K₂))
        i j)
    (d
        (Functor.obj
          (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
            (Functor.obj (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
          K₂)
        i j ≫
      Iso.hom
        ((fun (x_1 : ℤ) ↦
            (Iso.refl
                (X
                  (Functor.obj
                    (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
                      (Functor.obj
                        (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
                    K₂)
                  x_1) :
              Iso (C := HomologicalComplex.{v_3, u_3, 0} D (ComplexShape.up ℤ))
                (X
                  (Functor.obj
                    (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
                      (Functor.obj
                        (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
                    K₂)
                  x_1)
                (X
                  (Functor.obj
                    (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
                      (Functor.obj
                        (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
                    K₂)
                  x_1)))
          j))
have h : ∀ (i_1 : ℤ),
  Eq (α :=
    X
        (X
          (Functor.obj
            (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
              (Functor.obj (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x)
                K₁))
            K₂)
          i)
        i_1 ⟶
      X
        (X
          (Functor.obj (HomologicalComplex₂.shiftFunctor₁ D x)
            (Functor.obj (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ)) K₁)
              K₂))
          j)
        i_1)
    (Hom.f
      (Iso.hom
          ((fun (x_1 : ℤ) ↦
              (Iso.refl
                  (X
                    (Functor.obj
                      (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
                        (Functor.obj
                          (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
                      K₂)
                    x_1) :
                Iso (C := HomologicalComplex.{v_3, u_3, 0} D (ComplexShape.up ℤ))
                  (X
                    (Functor.obj
                      (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
                        (Functor.obj
                          (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
                      K₂)
                    x_1)
                  (X
                    (Functor.obj
                      (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
                        (Functor.obj
                          (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
                      K₂)
                    x_1)))
            i) ≫
        d
          (Functor.obj (HomologicalComplex₂.shiftFunctor₁ D x)
            (Functor.obj (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ)) K₁)
              K₂))
          i j)
      i_1)
    (Hom.f
      (d
          (Functor.obj
            (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
              (Functor.obj (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x)
                K₁))
            K₂)
          i j ≫
        Iso.hom
          ((fun (x_1 : ℤ) ↦
              (Iso.refl
                  (X
                    (Functor.obj
                      (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
                        (Functor.obj
                          (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
                      K₂)
                    x_1) :
                Iso (C := HomologicalComplex.{v_3, u_3, 0} D (ComplexShape.up ℤ))
                  (X
                    (Functor.obj
                      (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
                        (Functor.obj
                          (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
                      K₂)
                    x_1)
                  (X
                    (Functor.obj
                      (Functor.obj (F.mapBifunctorHomologicalComplex (ComplexShape.up ℤ) (ComplexShape.up ℤ))
                        (Functor.obj
                          (CategoryTheory.shiftFunctor (HomologicalComplex.{v_1, u_1, 0} C₁ (ComplexShape.up ℤ)) x) K₁))
                      K₂)
                    x_1)))
            j))
      i_1)
Tactic: ext