-- Mathlib/CategoryTheory/Triangulated/Functor.lean
/-- If `F : C ⥤ D` is a triangulated functor from a triangulated category, then `D`
is also triangulated if tuples of composables arrows in `D` can be lifted to `C`. -/
lemma isTriangulated_of_essSurj_mapComposableArrows_two
    (F : C ⥤ D) [F.CommShift ℤ] [F.IsTriangulated]
    [(F.mapComposableArrows 2).EssSurj] [IsTriangulated C] :
    IsTriangulated D

Proof state:
case octahedron_axiom
C : Type u_1
D : Type u_2
inst✝¹⁵ : Category.{u_3, u_1} C
inst✝¹⁴ : Category.{u_4, u_2} D
inst✝¹³ : HasShift C ℤ
inst✝¹² : HasShift D ℤ
inst✝¹¹ : HasZeroObject C
inst✝¹⁰ : HasZeroObject D
inst✝⁹ : Preadditive C
inst✝⁸ : Preadditive D
inst✝⁷ : ∀ (n : ℤ), (shiftFunctor C n).Additive
inst✝⁶ : ∀ (n : ℤ), (shiftFunctor D n).Additive
inst✝⁵ : Pretriangulated C
inst✝⁴ : Pretriangulated D
F : C ⥤ D
inst✝³ : F.CommShift ℤ
inst✝² : F.IsTriangulated
inst✝¹ : (F.mapComposableArrows 2).EssSurj
inst✝ : IsTriangulated C
⊢ ∀ {X₁ X₂ X₃ Z₁₂ Z₂₃ Z₁₃ : D} {u₁₂ : X₁ ⟶ X₂} {u₂₃ : X₂ ⟶ X₃} {u₁₃ : X₁ ⟶ X₃} (comm : u₁₂ ≫ u₂₃ = u₁₃) {v₁₂ : X₂ ⟶ Z₁₂}
    {w₁₂ : Z₁₂ ⟶ (shiftFunctor D 1).obj X₁} (h₁₂ : Triangle.mk u₁₂ v₁₂ w₁₂ ∈ distinguishedTriangles) {v₂₃ : X₃ ⟶ Z₂₃}
    {w₂₃ : Z₂₃ ⟶ (shiftFunctor D 1).obj X₂} (h₂₃ : Triangle.mk u₂₃ v₂₃ w₂₃ ∈ distinguishedTriangles) {v₁₃ : X₃ ⟶ Z₁₃}
    {w₁₃ : Z₁₃ ⟶ (shiftFunctor D 1).obj X₁} (h₁₃ : Triangle.mk u₁₃ v₁₃ w₁₃ ∈ distinguishedTriangles),
    Nonempty (Octahedron comm h₁₂ h₂₃ h₁₃)

Tactic:
intro Y₁ Y₂ Y₃ Z₁₂ Z₂₃ Z₁₃ u₁₂ u₂₃ u₁₃ comm v₁₂ w₁₂ h₁₂ v₂₃ w₂₃ h₂₃ v₁₃ w₁₃ h₁₃

Result:
case octahedron_axiom
C : Type u_1
D : Type u_2
inst✝¹⁵ : Category.{u_3, u_1} C
inst✝¹⁴ : Category.{u_4, u_2} D
inst✝¹³ : HasShift C ℤ
inst✝¹² : HasShift D ℤ
inst✝¹¹ : HasZeroObject C
inst✝¹⁰ : HasZeroObject D
inst✝⁹ : Preadditive C
inst✝⁸ : Preadditive D
inst✝⁷ : ∀ (n : ℤ), (shiftFunctor C n).Additive
inst✝⁶ : ∀ (n : ℤ), (shiftFunctor D n).Additive
inst✝⁵ : Pretriangulated C
inst✝⁴ : Pretriangulated D
F : C ⥤ D
inst✝³ : F.CommShift ℤ
inst✝² : F.IsTriangulated
inst✝¹ : (F.mapComposableArrows 2).EssSurj
inst✝ : IsTriangulated C
Y₁ Y₂ Y₃ Z₁₂ Z₂₃ Z₁₃ : D
u₁₂ : Y₁ ⟶ Y₂
u₂₃ : Y₂ ⟶ Y₃
u₁₃ : Y₁ ⟶ Y₃
comm : u₁₂ ≫ u₂₃ = u₁₃
v₁₂ : Y₂ ⟶ Z₁₂
w₁₂ : Z₁₂ ⟶ (shiftFunctor D 1).obj Y₁
h₁₂ : Triangle.mk u₁₂ v₁₂ w₁₂ ∈ distinguishedTriangles
v₂₃ : Y₃ ⟶ Z₂₃
w₂₃ : Z₂₃ ⟶ (shiftFunctor D 1).obj Y₂
h₂₃ : Triangle.mk u₂₃ v₂₃ w₂₃ ∈ distinguishedTriangles
v₁₃ : Y₃ ⟶ Z₁₃
w₁₃ : Z₁₃ ⟶ (shiftFunctor D 1).obj Y₁
h₁₃ : Triangle.mk u₁₃ v₁₃ w₁₃ ∈ distinguishedTriangles
⊢ Nonempty (Octahedron comm h₁₂ h₂₃ h₁₃)

Proof state:
case octahedron_axiom.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
C : Type u_1
D : Type u_2
inst✝¹⁵ : Category.{u_3, u_1} C
inst✝¹⁴ : Category.{u_4, u_2} D
inst✝¹³ : HasShift C ℤ
inst✝¹² : HasShift D ℤ
inst✝¹¹ : HasZeroObject C
inst✝¹⁰ : HasZeroObject D
inst✝⁹ : Preadditive C
inst✝⁸ : Preadditive D
inst✝⁷ : ∀ (n : ℤ), (shiftFunctor C n).Additive
inst✝⁶ : ∀ (n : ℤ), (shiftFunctor D n).Additive
inst✝⁵ : Pretriangulated C
inst✝⁴ : Pretriangulated D
F : C ⥤ D
inst✝³ : F.CommShift ℤ
inst✝² : F.IsTriangulated
inst✝¹ : (F.mapComposableArrows 2).EssSurj
inst✝ : IsTriangulated C
Y₁ Y₂ Y₃ Z₁₂ Z₂₃ Z₁₃ : D
u₁₂ : Y₁ ⟶ Y₂
u₂₃ : Y₂ ⟶ Y₃
u₁₃ : Y₁ ⟶ Y₃
comm : u₁₂ ≫ u₂₃ = u₁₃
v₁₂ : Y₂ ⟶ Z₁₂
w₁₂ : Z₁₂ ⟶ (shiftFunctor D 1).obj Y₁
h₁₂ : Triangle.mk u₁₂ v₁₂ w₁₂ ∈ distinguishedTriangles
v₂₃ : Y₃ ⟶ Z₂₃
w₂₃ : Z₂₃ ⟶ (shiftFunctor D 1).obj Y₂
h₂₃ : Triangle.mk u₂₃ v₂₃ w₂₃ ∈ distinguishedTriangles
v₁₃ : Y₃ ⟶ Z₁₃
w₁₃ : Z₁₃ ⟶ (shiftFunctor D 1).obj Y₁
h₁₃ : Triangle.mk u₁₃ v₁₃ w₁₃ ∈ distinguishedTriangles
X₁ X₂ X₃ : C
f : X₁ ⟶ X₂
g : X₂ ⟶ X₃
e : (F.mapComposableArrows 2).obj (ComposableArrows.mk₂ f g) ≅ ComposableArrows.mk₂ u₁₂ u₂₃
w✝² : C
w✝¹ : X₂ ⟶ w✝²
w✝ : w✝² ⟶ (shiftFunctor C 1).obj X₁
h₁₂' : Triangle.mk f w✝¹ w✝ ∈ distinguishedTriangles
⊢ Nonempty (Octahedron comm h₁₂ h₂₃ h₁₃)

Tactic:
obtain ⟨_, _, _, h₂₃'⟩ := distinguished_cocone_triangle g

Result:
case octahedron_axiom.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
C : Type u_1
D : Type u_2
inst✝¹⁵ : Category.{u_3, u_1} C
inst✝¹⁴ : Category.{u_4, u_2} D
inst✝¹³ : HasShift C ℤ
inst✝¹² : HasShift D ℤ
inst✝¹¹ : HasZeroObject C
inst✝¹⁰ : HasZeroObject D
inst✝⁹ : Preadditive C
inst✝⁸ : Preadditive D
inst✝⁷ : ∀ (n : ℤ), (shiftFunctor C n).Additive
inst✝⁶ : ∀ (n : ℤ), (shiftFunctor D n).Additive
inst✝⁵ : Pretriangulated C
inst✝⁴ : Pretriangulated D
F : C ⥤ D
inst✝³ : F.CommShift ℤ
inst✝² : F.IsTriangulated
inst✝¹ : (F.mapComposableArrows 2).EssSurj
inst✝ : IsTriangulated C
Y₁ Y₂ Y₃ Z₁₂ Z₂₃ Z₁₃ : D
u₁₂ : Y₁ ⟶ Y₂
u₂₃ : Y₂ ⟶ Y₃
u₁₃ : Y₁ ⟶ Y₃
comm : u₁₂ ≫ u₂₃ = u₁₃
v₁₂ : Y₂ ⟶ Z₁₂
w₁₂ : Z₁₂ ⟶ (shiftFunctor D 1).obj Y₁
h₁₂ : Triangle.mk u₁₂ v₁₂ w₁₂ ∈ distinguishedTriangles
v₂₃ : Y₃ ⟶ Z₂₃
w₂₃ : Z₂₃ ⟶ (shiftFunctor D 1).obj Y₂
h₂₃ : Triangle.mk u₂₃ v₂₃ w₂₃ ∈ distinguishedTriangles
v₁₃ : Y₃ ⟶ Z₁₃
w₁₃ : Z₁₃ ⟶ (shiftFunctor D 1).obj Y₁
h₁₃ : Triangle.mk u₁₃ v₁₃ w₁₃ ∈ distinguishedTriangles
X₁ X₂ X₃ : C
f : X₁ ⟶ X₂
g : X₂ ⟶ X₃
e : (F.mapComposableArrows 2).obj (ComposableArrows.mk₂ f g) ≅ ComposableArrows.mk₂ u₁₂ u₂₃
w✝⁵ : C
w✝⁴ : X₂ ⟶ w✝⁵
w✝³ : w✝⁵ ⟶ (shiftFunctor C 1).obj X₁
h₁₂' : Triangle.mk f w✝⁴ w✝³ ∈ distinguishedTriangles
w✝² : C
w✝¹ : X₃ ⟶ w✝²
w✝ : w✝² ⟶ (shiftFunctor C 1).obj X₂
h₂₃' : Triangle.mk g w✝¹ w✝ ∈ distinguishedTriangles
⊢ Nonempty (Octahedron comm h₁₂ h₂₃ h₁₃)