-- Mathlib/CategoryTheory/Triangulated/Pretriangulated.lean
lemma mor₃_eq_zero_iff_epi₂ : T.mor₃ = 0 ↔ Epi T.mor₂

Proof state:
case mp
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
h : T.mor₃ = 0
X : C
g : T.obj₃ ⟶ X
hg : T.mor₂ ≫ g = 0
⊢ g = 0

Tactic:
obtain ⟨f, rfl⟩ := yoneda_exact₃ T hT g hg

Result:
case mp.intro
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
h : T.mor₃ = 0
X : C
f : (CategoryTheory.shiftFunctor C 1).obj T.obj₁ ⟶ X
hg : T.mor₂ ≫ T.mor₃ ≫ f = 0
⊢ T.mor₃ ≫ f = 0

Proof state:
case mp.intro
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
h : T.mor₃ = 0
X : C
f : (CategoryTheory.shiftFunctor C 1).obj T.obj₁ ⟶ X
hg : T.mor₂ ≫ T.mor₃ ≫ f = 0
⊢ T.mor₃ ≫ f = 0

Tactic:
rw [h, zero_comp]

Result:
no goals

Proof state:
case mp
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
h : T.mor₃ = 0
⊢ Epi T.mor₂

Tactic:
rw [epi_iff_cancel_zero]

Result:
case mp
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
h : T.mor₃ = 0
⊢ ∀ (R : C) (g : T.obj₃ ⟶ R), T.mor₂ ≫ g = 0 → g = 0

Proof state:
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
⊢ T.mor₃ = 0 ↔ Epi T.mor₂

Tactic:
constructor

Result:
case mp
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
⊢ T.mor₃ = 0 → Epi T.mor₂

case mpr
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
⊢ Epi T.mor₂ → T.mor₃ = 0

Proof state:
case mp
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
h : T.mor₃ = 0
⊢ ∀ (R : C) (g : T.obj₃ ⟶ R), T.mor₂ ≫ g = 0 → g = 0

Tactic:
intro X g hg

Result:
case mp
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
h : T.mor₃ = 0
X : C
g : T.obj₃ ⟶ X
hg : T.mor₂ ≫ g = 0
⊢ g = 0

Proof state:
case mp
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
⊢ T.mor₃ = 0 → Epi T.mor₂

Tactic:
intro h

Result:
case mp
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
h : T.mor₃ = 0
⊢ Epi T.mor₂

Proof state:
case mpr
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
a✝ : Epi T.mor₂
⊢ T.mor₃ = 0

Tactic:
rw [← cancel_epi T.mor₂, comp_distTriang_mor_zero₂₃ _ hT, comp_zero]

Result:
no goals

Proof state:
case mpr
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
⊢ Epi T.mor₂ → T.mor₃ = 0

Tactic:
intro

Result:
case mpr
C : Type u
inst✝⁴ : Category.{v, u} C
inst✝³ : HasZeroObject C
inst✝² : HasShift C ℤ
inst✝¹ : Preadditive C
inst✝ : ∀ (n : ℤ), (CategoryTheory.shiftFunctor C n).Additive
hC : Pretriangulated C
T : Triangle C
hT : T ∈ distinguishedTriangles
a✝ : Epi T.mor₂
⊢ T.mor₃ = 0