-- Mathlib/Algebra/Homology/HomologicalComplex.lean
theorem ext {C₁ C₂ : HomologicalComplex V c} (h_X : C₁.X = C₂.X)
    (h_d :
      ∀ i j : ι,
        c.Rel i j → C₁.d i j ≫ eqToHom (congr_fun h_X j) = eqToHom (congr_fun h_X i) ≫ C₂.d i j) :
    C₁ = C₂

Proof state:
case mk.mk
ι : Type u_1
V : Type u
inst✝¹ : Category.{v, u} V
inst✝ : HasZeroMorphisms V
c : ComplexShape ι
X₁ : ι → V
d₁ : (i j : ι) → X₁ i ⟶ X₁ j
s₁ : ∀ (i j : ι), ¬c.Rel i j → d₁ i j = 0
h₁ : ∀ (i j k : ι), c.Rel i j → c.Rel j k → d₁ i j ≫ d₁ j k = 0
X₂ : ι → V
d₂ : (i j : ι) → X₂ i ⟶ X₂ j
s₂ : ∀ (i j : ι), ¬c.Rel i j → d₂ i j = 0
h₂ : ∀ (i j k : ι), c.Rel i j → c.Rel j k → d₂ i j ≫ d₂ j k = 0
h_X : X₁ = X₂
h_d :
  ∀ (i j : ι),
    c.Rel i j →
      { X := X₁, d := d₁, shape := s₁, d_comp_d' := h₁ }.d i j ≫ eqToHom ⋯ =
        eqToHom ⋯ ≫ { X := X₂, d := d₂, shape := s₂, d_comp_d' := h₂ }.d i j
⊢ { X := X₁, d := d₁, shape := s₁, d_comp_d' := h₁ } = { X := X₂, d := d₂, shape := s₂, d_comp_d' := h₂ }

Tactic:
subst h_X

Result:
case mk.mk
ι : Type u_1
V : Type u
inst✝¹ : Category.{v, u} V
inst✝ : HasZeroMorphisms V
c : ComplexShape ι
X₁ : ι → V
d₁ : (i j : ι) → X₁ i ⟶ X₁ j
s₁ : ∀ (i j : ι), ¬c.Rel i j → d₁ i j = 0
h₁ : ∀ (i j k : ι), c.Rel i j → c.Rel j k → d₁ i j ≫ d₁ j k = 0
d₂ : (i j : ι) → X₁ i ⟶ X₁ j
s₂ : ∀ (i j : ι), ¬c.Rel i j → d₂ i j = 0
h₂ : ∀ (i j k : ι), c.Rel i j → c.Rel j k → d₂ i j ≫ d₂ j k = 0
h_d :
  ∀ (i j : ι),
    c.Rel i j →
      { X := X₁, d := d₁, shape := s₁, d_comp_d' := h₁ }.d i j ≫ eqToHom ⋯ =
        eqToHom ⋯ ≫ { X := X₁, d := d₂, shape := s₂, d_comp_d' := h₂ }.d i j
⊢ { X := X₁, d := d₁, shape := s₁, d_comp_d' := h₁ } = { X := X₁, d := d₂, shape := s₂, d_comp_d' := h₂ }

Proof state:
case mk
ι : Type u_1
V : Type u
inst✝¹ : Category.{v, u} V
inst✝ : HasZeroMorphisms V
c : ComplexShape ι
C₂ : HomologicalComplex V c
X₁ : ι → V
d₁ : (i j : ι) → X₁ i ⟶ X₁ j
s₁ : ∀ (i j : ι), ¬c.Rel i j → d₁ i j = 0
h₁ : ∀ (i j k : ι), c.Rel i j → c.Rel j k → d₁ i j ≫ d₁ j k = 0
h_X : { X := X₁, d := d₁, shape := s₁, d_comp_d' := h₁ }.X = C₂.X
h_d :
  ∀ (i j : ι), c.Rel i j → { X := X₁, d := d₁, shape := s₁, d_comp_d' := h₁ }.d i j ≫ eqToHom ⋯ = eqToHom ⋯ ≫ C₂.d i j
⊢ { X := X₁, d := d₁, shape := s₁, d_comp_d' := h₁ } = C₂

Tactic:
obtain ⟨X₂, d₂, s₂, h₂⟩ := C₂

Result:
case mk.mk
ι : Type u_1
V : Type u
inst✝¹ : Category.{v, u} V
inst✝ : HasZeroMorphisms V
c : ComplexShape ι
X₁ : ι → V
d₁ : (i j : ι) → X₁ i ⟶ X₁ j
s₁ : ∀ (i j : ι), ¬c.Rel i j → d₁ i j = 0
h₁ : ∀ (i j k : ι), c.Rel i j → c.Rel j k → d₁ i j ≫ d₁ j k = 0
X₂ : ι → V
d₂ : (i j : ι) → X₂ i ⟶ X₂ j
s₂ : ∀ (i j : ι), ¬c.Rel i j → d₂ i j = 0
h₂ : ∀ (i j k : ι), c.Rel i j → c.Rel j k → d₂ i j ≫ d₂ j k = 0
h_X : { X := X₁, d := d₁, shape := s₁, d_comp_d' := h₁ }.X = { X := X₂, d := d₂, shape := s₂, d_comp_d' := h₂ }.X
h_d :
  ∀ (i j : ι),
    c.Rel i j →
      { X := X₁, d := d₁, shape := s₁, d_comp_d' := h₁ }.d i j ≫ eqToHom ⋯ =
        eqToHom ⋯ ≫ { X := X₂, d := d₂, shape := s₂, d_comp_d' := h₂ }.d i j
⊢ { X := X₁, d := d₁, shape := s₁, d_comp_d' := h₁ } = { X := X₂, d := d₂, shape := s₂, d_comp_d' := h₂ }

Proof state:
case neg
ι : Type u_1
V : Type u
inst✝¹ : Category.{v, u} V
inst✝ : HasZeroMorphisms V
c : ComplexShape ι
X₁ : ι → V
d₁ : (i j : ι) → X₁ i ⟶ X₁ j
s₁ : ∀ (i j : ι), ¬c.Rel i j → d₁ i j = 0
h₁ : ∀ (i j k : ι), c.Rel i j → c.Rel j k → d₁ i j ≫ d₁ j k = 0
d₂ : (i j : ι) → X₁ i ⟶ X₁ j
s₂ : ∀ (i j : ι), ¬c.Rel i j → d₂ i j = 0
h₂ : ∀ (i j k : ι), c.Rel i j → c.Rel j k → d₂ i j ≫ d₂ j k = 0
h_d :
  ∀ (i j : ι),
    c.Rel i j →
      { X := X₁, d := d₁, shape := s₁, d_comp_d' := h₁ }.d i j ≫ eqToHom ⋯ =
        eqToHom ⋯ ≫ { X := X₁, d := d₂, shape := s₂, d_comp_d' := h₂ }.d i j
i j : ι
hij : ¬c.Rel i j
⊢ d₁ i j = d₂ i j

Tactic:
rw [s₁ i j hij, s₂ i j hij]

Result:
no goals