-- Lean 4 proof step from Mathlib.CategoryTheory.Triangulated.Triangulated (goal)
Goal:
C : Type u_1
inst : Category.{v_1, u_1} C
inst_1 : Preadditive.{v_1, u_1} C
inst_2 : HasZeroObject.{v_1, u_1} C
inst_3 : HasShift.{v_1, u_1, 0} C ℤ
inst_4 : ∀ (n : ℤ), Functor.Additive.{v_1, u_1, v_1, u_1} (shiftFunctor C n)
inst_5 : Pretriangulated.{v_1, u_1} C
h :
  ∀ ⦃X₁' X₂' X₃' : C⦄ (u₁₂' : X₁' ⟶ X₂') (u₂₃' : X₂' ⟶ X₃'),
    ∃ (X₁ : C) (X₂ : C) (X₃ : C) (Z₁₂ : C) (Z₂₃ : C) (Z₁₃ : C) (u₁₂ : X₁ ⟶ X₂) (u₂₃ : X₂ ⟶ X₃) (e₁ : X₁' ≅ X₁) (e₂ :
      X₂' ≅ X₂) (e₃ : X₃' ≅ X₃) (_ : u₁₂' ≫ e₂.hom = e₁.hom ≫ u₁₂) (_ : u₂₃' ≫ e₃.hom = e₂.hom ≫ u₂₃) (v₁₂ : X₂ ⟶ Z₁₂)
      (w₁₂ : Z₁₂ ⟶ Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C (A := ℤ) 1) X₁) (h₁₂ :
      Membership.mem (γ := Set (Triangle.{v_1, u_1} C)) (@distinguishedTriangles _ _ inst_2 _ inst_1 inst_4 _)
        (Triangle.mk u₁₂ v₁₂ w₁₂))
      (v₂₃ : X₃ ⟶ Z₂₃) (w₂₃ : Z₂₃ ⟶ Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C (A := ℤ) 1) X₂) (h₂₃ :
      Membership.mem (γ := Set (Triangle.{v_1, u_1} C)) (@distinguishedTriangles _ _ inst_2 _ inst_1 inst_4 _)
        (Triangle.mk u₂₃ v₂₃ w₂₃))
      (v₁₃ : X₃ ⟶ Z₁₃) (w₁₃ : Z₁₃ ⟶ Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C (A := ℤ) 1) X₁) (h₁₃ :
      Membership.mem (γ := Set (Triangle.{v_1, u_1} C)) (@distinguishedTriangles _ _ inst_2 _ inst_1 inst_4 _)
        (Triangle.mk (u₁₂ ≫ u₂₃) v₁₃ w₁₃)),
      Nonempty.{v_1 + 1} (Octahedron ⋯ h₁₂ h₂₃ h₁₃)
X₁' X₂' X₃' Z₁₂' Z₂₃' Z₁₃' : C
u₁₂' : X₁' ⟶ X₂'
u₂₃' : X₂' ⟶ X₃'
u₁₃' : X₁' ⟶ X₃'
comm' : u₁₂' ≫ u₂₃' = u₁₃'
v₁₂' : X₂' ⟶ Z₁₂'
w₁₂' : Z₁₂' ⟶ Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C (A := ℤ) 1) X₁'
h₁₂' :
  Membership.mem (γ := Set (Triangle.{v_1, u_1} C)) (@distinguishedTriangles _ _ inst_2 _ inst_1 inst_4 _)
    (Triangle.mk u₁₂' v₁₂' w₁₂')
v₂₃' : X₃' ⟶ Z₂₃'
w₂₃' : Z₂₃' ⟶ Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C (A := ℤ) 1) X₂'
h₂₃' :
  Membership.mem (γ := Set (Triangle.{v_1, u_1} C)) (@distinguishedTriangles _ _ inst_2 _ inst_1 inst_4 _)
    (Triangle.mk u₂₃' v₂₃' w₂₃')
v₁₃' : X₃' ⟶ Z₁₃'
w₁₃' : Z₁₃' ⟶ Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C (A := ℤ) 1) X₁'
h₁₃' :
  Membership.mem (γ := Set (Triangle.{v_1, u_1} C)) (@distinguishedTriangles _ _ inst_2 _ inst_1 inst_4 _)
    (Triangle.mk u₁₃' v₁₃' w₁₃')
⊢ Nonempty.{v_1 + 1} (Octahedron comm' h₁₂' h₂₃' h₁₃')
have obtain : ∀ (X₁ X₂ X₃ Z₁₂ Z₂₃ Z₁₃ : C) (u₁₂ : X₁ ⟶ X₂) (u₂₃ : X₂ ⟶ X₃) (e₁ : X₁' ≅ X₁) (e₂ : X₂' ≅ X₂) (e₃ : X₃' ≅ X₃)
  (comm₁₂ : u₁₂' ≫ e₂.hom = e₁.hom ≫ u₁₂) (comm₂₃ : u₂₃' ≫ e₃.hom = e₂.hom ≫ u₂₃) (v₁₂ : X₂ ⟶ Z₁₂)
  (w₁₂ : Z₁₂ ⟶ Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C (A := ℤ) 1) X₁)
  (h₁₂ :
    Membership.mem (γ := Set (Triangle.{v_1, u_1} C)) (@distinguishedTriangles _ _ inst_2 _ inst_1 inst_4 _)
      (Triangle.mk u₁₂ v₁₂ w₁₂))
  (v₂₃ : X₃ ⟶ Z₂₃) (w₂₃ : Z₂₃ ⟶ Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C (A := ℤ) 1) X₂)
  (h₂₃ :
    Membership.mem (γ := Set (Triangle.{v_1, u_1} C)) (@distinguishedTriangles _ _ inst_2 _ inst_1 inst_4 _)
      (Triangle.mk u₂₃ v₂₃ w₂₃))
  (v₁₃ : X₃ ⟶ Z₁₃) (w₁₃ : Z₁₃ ⟶ Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C (A := ℤ) 1) X₁)
  (h₁₃ :
    Membership.mem (γ := Set (Triangle.{v_1, u_1} C)) (@distinguishedTriangles _ _ inst_2 _ inst_1 inst_4 _)
      (Triangle.mk (u₁₂ ≫ u₂₃) v₁₃ w₁₃))
  (H : Nonempty.{v_1 + 1} (Octahedron sorry h₁₂ h₂₃ h₁₃)), Nonempty.{v_1 + 1} (Octahedron comm' h₁₂' h₂₃' h₁₃')
Tactic: obtain ⟨X₁, X₂, X₃, Z₁₂, Z₂₃, Z₁₃, u₁₂, u₂₃, e₁, e₂, e₃, comm₁₂, comm₂₃,
      v₁₂, w₁₂, h₁₂, v₂₃, w₂₃, h₂₃, v₁₃, w₁₃, h₁₃, H⟩ := h u₁₂' u₂₃'