-- Lean 4 proof step from Mathlib.CategoryTheory.Triangulated.Yoneda (goal)
Goal:
C : Type u_1
inst : Category.{v_1, u_1} C
inst_1 : Preadditive.{v_1, u_1} C
inst_2 : HasShift.{v_1, u_1, 0} C ℤ
inst_3 : ∀ (n : ℤ), Functor.Additive.{v_1, u_1, v_1, u_1} (shiftFunctor C n)
B : C
a : ℤ
⊢ Eq (α :=
    Iso (C := Functor.{v_1, v_1, u_1, v_1 + 1} Cᵒᵖ AddCommGrpCat)
      (shiftFunctor Cᵒᵖ (A := ℤ) 0 ⋙
        Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B))
      (Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B)))
    (NatIso.ofComponents
      (fun (A : Cᵒᵖ) ↦
        AddEquiv.toAddCommGrpIso
          (let __Equiv :
            (↑(Functor.obj
                    (shiftFunctor Cᵒᵖ (A := ℤ) 0 ⋙
                      Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B))
                    A) :
                Type v_1) ≃
              (↑(Functor.obj (Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B))
                    A) :
                Type v_1) :=
            Equiv.trans Quiver.Hom.opEquiv (Equiv.symm (ShiftedHom.opEquiv' 0 a a ⋯));
          { toEquiv := __Equiv, map_add' := ⋯ }))
      ⋯)
    (Functor.isoWhiskerRight (shiftFunctorZero Cᵒᵖ ℤ)
        (Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B)) ≪≫
      Functor.leftUnitor (Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B)))
have w : ∀ (x : Cᵒᵖ)
  (x_1 :
    (↑(Functor.obj
          (shiftFunctor Cᵒᵖ (A := ℤ) 0 ⋙
            Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B))
          x) :
      Type v_1)),
  Eq (α :=
    (↑(Functor.obj (Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B)) x) :
      Type v_1))
    ((AddCommGrpCat.Hom.hom
            (NatTrans.app
              (Iso.hom
                (NatIso.ofComponents
                  (fun (A : Cᵒᵖ) ↦
                    AddEquiv.toAddCommGrpIso
                      (let __Equiv :
                        (↑(Functor.obj
                                (shiftFunctor Cᵒᵖ (A := ℤ) 0 ⋙
                                  Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B))
                                A) :
                            Type v_1) ≃
                          (↑(Functor.obj
                                (Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B))
                                A) :
                            Type v_1) :=
                        Equiv.trans Quiver.Hom.opEquiv (Equiv.symm (ShiftedHom.opEquiv' 0 a a sorry));
                      { toEquiv := __Equiv, map_add' := sorry }))
                  sorry))
              x) :
          (a_1 : (↑((shiftFunctor Cᵒᵖ 0 ⋙ preadditiveYoneda.obj ((shiftFunctor C a).obj B)).obj x) : Type v_1)) →
            (↑((preadditiveYoneda.obj ((shiftFunctor C a).obj B)).obj x) : Type v_1))
        x_1 :
      (↑(Functor.obj (Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B)) x) :
        Type v_1))
    ((AddCommGrpCat.Hom.hom
            (NatTrans.app
              (Iso.hom
                (Functor.isoWhiskerRight (shiftFunctorZero Cᵒᵖ ℤ)
                    (Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B)) ≪≫
                  Functor.leftUnitor
                    (Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B))))
              x) :
          (a_1 : (↑((shiftFunctor Cᵒᵖ 0 ⋙ preadditiveYoneda.obj ((shiftFunctor C a).obj B)).obj x) : Type v_1)) →
            (↑((preadditiveYoneda.obj ((shiftFunctor C a).obj B)).obj x) : Type v_1))
        x_1 :
      (↑(Functor.obj (Functor.obj preadditiveYoneda (Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C a) B)) x) :
        Type v_1))
Tactic: ext