-- Mathlib/Algebra/Homology/ShortComplex/ConcreteCategory.lean
/-- This lemma allows the computation of the connecting homomorphism
`D.δ` when `D : SnakeInput C` and `C` is a concrete category. -/
lemma δ_apply (x₃ : ToType (D.L₀.X₃)) (x₂ : ToType (D.L₁.X₂)) (x₁ : ToType (D.L₂.X₁))
    (h₂ : D.L₁.g x₂ = D.v₀₁.τ₃ x₃) (h₁ : D.L₂.f x₁ = D.v₁₂.τ₂ x₂) :
    D.δ x₃ = D.v₂₃.τ₁ x₁

Proof state:
C : Type u
inst✝⁶ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝⁵ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝⁴ : ConcreteCategory C FC
inst✝³ : HasForget₂ C Ab
inst✝² : Abelian C
inst✝¹ : (forget₂ C Ab).Additive
inst✝ : (forget₂ C Ab).PreservesHomology
D : SnakeInput C
x₃ : ToType D.L₀.X₃
x₂ : ToType D.L₁.X₂
x₁ : ToType D.L₂.X₁
h₂ : (ConcreteCategory.hom D.L₁.g) x₂ = (ConcreteCategory.hom D.v₀₁.τ₃) x₃
h₁ : (ConcreteCategory.hom D.L₂.f) x₁ = (ConcreteCategory.hom D.v₁₂.τ₂) x₂
this✝ : PreservesFiniteLimits (forget₂ C Ab)
this : PreservesFiniteLimits (forget C)
eq :
  (ConcreteCategory.hom D.δ)
      ((ConcreteCategory.hom (pullback.snd D.L₁.g D.v₀₁.τ₃)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂)) =
    (ConcreteCategory.hom D.v₂₃.τ₁) ((ConcreteCategory.hom D.φ₁) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂))
eq₁ : (ConcreteCategory.hom (pullback.fst D.L₁.g D.v₀₁.τ₃)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) = x₂
eq₂ : (ConcreteCategory.hom (pullback.snd D.L₁.g D.v₀₁.τ₃)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) = x₃
⊢ (ConcreteCategory.hom D.δ) x₃ = (ConcreteCategory.hom D.v₂₃.τ₁) x₁

Tactic:
rw [eq₂] at eq

Result:
C : Type u
inst✝⁶ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝⁵ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝⁴ : ConcreteCategory C FC
inst✝³ : HasForget₂ C Ab
inst✝² : Abelian C
inst✝¹ : (forget₂ C Ab).Additive
inst✝ : (forget₂ C Ab).PreservesHomology
D : SnakeInput C
x₃ : ToType D.L₀.X₃
x₂ : ToType D.L₁.X₂
x₁ : ToType D.L₂.X₁
h₂ : (ConcreteCategory.hom D.L₁.g) x₂ = (ConcreteCategory.hom D.v₀₁.τ₃) x₃
h₁ : (ConcreteCategory.hom D.L₂.f) x₁ = (ConcreteCategory.hom D.v₁₂.τ₂) x₂
this✝ : PreservesFiniteLimits (forget₂ C Ab)
this : PreservesFiniteLimits (forget C)
eq :
  (ConcreteCategory.hom D.δ) x₃ =
    (ConcreteCategory.hom D.v₂₃.τ₁) ((ConcreteCategory.hom D.φ₁) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂))
eq₁ : (ConcreteCategory.hom (pullback.fst D.L₁.g D.v₀₁.τ₃)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) = x₂
eq₂ : (ConcreteCategory.hom (pullback.snd D.L₁.g D.v₀₁.τ₃)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) = x₃
⊢ (ConcreteCategory.hom D.δ) x₃ = (ConcreteCategory.hom D.v₂₃.τ₁) x₁

Proof state:
case a
C : Type u
inst✝⁶ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝⁵ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝⁴ : ConcreteCategory C FC
inst✝³ : HasForget₂ C Ab
inst✝² : Abelian C
inst✝¹ : (forget₂ C Ab).Additive
inst✝ : (forget₂ C Ab).PreservesHomology
D : SnakeInput C
x₃ : ToType D.L₀.X₃
x₂ : ToType D.L₁.X₂
x₁ : ToType D.L₂.X₁
h₂ : (ConcreteCategory.hom D.L₁.g) x₂ = (ConcreteCategory.hom D.v₀₁.τ₃) x₃
h₁ : (ConcreteCategory.hom D.L₂.f) x₁ = (ConcreteCategory.hom D.v₁₂.τ₂) x₂
this✝ : PreservesFiniteLimits (forget₂ C Ab)
this : PreservesFiniteLimits (forget C)
eq :
  (ConcreteCategory.hom D.δ) x₃ =
    (ConcreteCategory.hom D.v₂₃.τ₁) ((ConcreteCategory.hom D.φ₁) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂))
eq₁ : (ConcreteCategory.hom (pullback.fst D.L₁.g D.v₀₁.τ₃)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) = x₂
eq₂ : (ConcreteCategory.hom (pullback.snd D.L₁.g D.v₀₁.τ₃)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) = x₃
⊢ (ConcreteCategory.hom D.φ₂) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) = (ConcreteCategory.hom D.L₂.f) x₁

Tactic:
dsimp [φ₂]

Result:
case a
C : Type u
inst✝⁶ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝⁵ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝⁴ : ConcreteCategory C FC
inst✝³ : HasForget₂ C Ab
inst✝² : Abelian C
inst✝¹ : (forget₂ C Ab).Additive
inst✝ : (forget₂ C Ab).PreservesHomology
D : SnakeInput C
x₃ : ToType D.L₀.X₃
x₂ : ToType D.L₁.X₂
x₁ : ToType D.L₂.X₁
h₂ : (ConcreteCategory.hom D.L₁.g) x₂ = (ConcreteCategory.hom D.v₀₁.τ₃) x₃
h₁ : (ConcreteCategory.hom D.L₂.f) x₁ = (ConcreteCategory.hom D.v₁₂.τ₂) x₂
this✝ : PreservesFiniteLimits (forget₂ C Ab)
this : PreservesFiniteLimits (forget C)
eq :
  (ConcreteCategory.hom D.δ) x₃ =
    (ConcreteCategory.hom D.v₂₃.τ₁) ((ConcreteCategory.hom D.φ₁) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂))
eq₁ : (ConcreteCategory.hom (pullback.fst D.L₁.g D.v₀₁.τ₃)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) = x₂
eq₂ : (ConcreteCategory.hom (pullback.snd D.L₁.g D.v₀₁.τ₃)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) = x₃
⊢ (ConcreteCategory.hom (pullback.fst D.L₁.g D.v₀₁.τ₃ ≫ D.v₁₂.τ₂)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) =
    (ConcreteCategory.hom D.L₂.f) x₁

Proof state:
C : Type u
inst✝⁶ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝⁵ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝⁴ : ConcreteCategory C FC
inst✝³ : HasForget₂ C Ab
inst✝² : Abelian C
inst✝¹ : (forget₂ C Ab).Additive
inst✝ : (forget₂ C Ab).PreservesHomology
D : SnakeInput C
x₃ : ToType D.L₀.X₃
x₂ : ToType D.L₁.X₂
x₁ : ToType D.L₂.X₁
h₂ : (ConcreteCategory.hom D.L₁.g) x₂ = (ConcreteCategory.hom D.v₀₁.τ₃) x₃
h₁ : (ConcreteCategory.hom D.L₂.f) x₁ = (ConcreteCategory.hom D.v₁₂.τ₂) x₂
this✝ : PreservesFiniteLimits (forget₂ C Ab)
this : PreservesFiniteLimits (forget C)
eq :
  (ConcreteCategory.hom (pullback.snd D.L₁.g D.v₀₁.τ₃ ≫ D.δ)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) =
    (ConcreteCategory.hom (D.φ₁ ≫ D.v₂₃.τ₁)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂)
⊢ (ConcreteCategory.hom D.δ) x₃ = (ConcreteCategory.hom D.v₂₃.τ₁) x₁

Tactic:
have eq₁ := Concrete.pullbackMk_fst D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂

Result:
C : Type u
inst✝⁶ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝⁵ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝⁴ : ConcreteCategory C FC
inst✝³ : HasForget₂ C Ab
inst✝² : Abelian C
inst✝¹ : (forget₂ C Ab).Additive
inst✝ : (forget₂ C Ab).PreservesHomology
D : SnakeInput C
x₃ : ToType D.L₀.X₃
x₂ : ToType D.L₁.X₂
x₁ : ToType D.L₂.X₁
h₂ : (ConcreteCategory.hom D.L₁.g) x₂ = (ConcreteCategory.hom D.v₀₁.τ₃) x₃
h₁ : (ConcreteCategory.hom D.L₂.f) x₁ = (ConcreteCategory.hom D.v₁₂.τ₂) x₂
this✝ : PreservesFiniteLimits (forget₂ C Ab)
this : PreservesFiniteLimits (forget C)
eq :
  (ConcreteCategory.hom (pullback.snd D.L₁.g D.v₀₁.τ₃ ≫ D.δ)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) =
    (ConcreteCategory.hom (D.φ₁ ≫ D.v₂₃.τ₁)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂)
eq₁ : (ConcreteCategory.hom (pullback.fst D.L₁.g D.v₀₁.τ₃)) (Concrete.pullbackMk D.L₁.g D.v₀₁.τ₃ x₂ x₃ h₂) = x₂
⊢ (ConcreteCategory.hom D.δ) x₃ = (ConcreteCategory.hom D.v₂₃.τ₁) x₁