Module: Mathlib.AlgebraicTopology.SimplicialSet.NerveAdjunction

Goal state before:
pp: case refine_1
X Y : Truncated 2
f₀ : X.obj (op { obj := ⦋0⦌, property := _proof_11 }) → Y.obj (op { obj := ⦋0⦌, property := _proof_11 })
f₁ : X.obj (op { obj := ⦋1⦌, property := _proof_12 }) → Y.obj (op { obj := ⦋1⦌, property := _proof_12 })
hδ₁ :
  ∀ (x : X.obj (op { obj := ⦋1⦌, property := _proof_12 })),
    f₀ ((ConcreteCategory.hom (X.map (δ₂ 1 _proof_11 _proof_13).op)) x) =
      (ConcreteCategory.hom (Y.map (δ₂ 1 _proof_11 _proof_13).op)) (f₁ x)
hδ₀ :
  ∀ (x : X.obj (op { obj := ⦋1⦌, property := _proof_12 })),
    f₀ ((ConcreteCategory.hom (X.map (δ₂ 0 _proof_11 _proof_13).op)) x) =
      (ConcreteCategory.hom (Y.map (δ₂ 0 _proof_11 _proof_13).op)) (f₁ x)
H :
  ∀ (x : X.obj (op { obj := ⦋2⦌, property := _proof_14 })) (y : Y.obj (op { obj := ⦋2⦌, property := _proof_14 })),
    f₁ ((ConcreteCategory.hom (X.map (δ₂ 2 _proof_12 _proof_15).op)) x) =
        (ConcreteCategory.hom (Y.map (δ₂ 2 _proof_12 _proof_15).op)) y →
      f₁ ((ConcreteCategory.hom (X.map (δ₂ 0 _proof_12 _proof_15).op)) x) =
          (ConcreteCategory.hom (Y.map (δ₂ 0 _proof_12 _proof_15).op)) y →
        f₁ ((ConcreteCategory.hom (X.map (δ₂ 1 _proof_12 _proof_15).op)) x) =
          (ConcreteCategory.hom (Y.map (δ₂ 1 _proof_12 _proof_15).op)) y
hσ :
  ∀ (x : X.obj (op { obj := ⦋0⦌, property := _proof_11 })),
    f₁ ((ConcreteCategory.hom (X.map (σ₂ 0 _proof_13 _proof_11).op)) x) =
      (ConcreteCategory.hom (Y.map (σ₂ 0 _proof_13 _proof_11).op)) (f₀ x)
hY : Y.StrictSegal
n : ℕ
hn : n < 2
i : Fin (n + 2)
⊢ naturalityProperty f₀ f₁ hδ₁ hδ₀ hY (δ 2 i ⋯ ⋯)

pp: case refine_2
X Y : Truncated 2
f₀ : X.obj (op { obj := ⦋0⦌, property := _proof_11 }) → Y.obj (op { obj := ⦋0⦌, property := _proof_11 })
f₁ : X.obj (op { obj := ⦋1⦌, property := _proof_12 }) → Y.obj (op { obj := ⦋1⦌, property := _proof_12 })
hδ₁ :
  ∀ (x : X.obj (op { obj := ⦋1⦌, property := _proof_12 })),
    f₀ ((ConcreteCategory.hom (X.map (δ₂ 1 _proof_11 _proof_13).op)) x) =
      (ConcreteCategory.hom (Y.map (δ₂ 1 _proof_11 _proof_13).op)) (f₁ x)
hδ₀ :
  ∀ (x : X.obj (op { obj := ⦋1⦌, property := _proof_12 })),
    f₀ ((ConcreteCategory.hom (X.map (δ₂ 0 _proof_11 _proof_13).op)) x) =
      (ConcreteCategory.hom (Y.map (δ₂ 0 _proof_11 _proof_13).op)) (f₁ x)
H :
  ∀ (x : X.obj (op { obj := ⦋2⦌, property := _proof_14 })) (y : Y.obj (op { obj := ⦋2⦌, property := _proof_14 })),
    f₁ ((ConcreteCategory.hom (X.map (δ₂ 2 _proof_12 _proof_15).op)) x) =
        (ConcreteCategory.hom (Y.map (δ₂ 2 _proof_12 _proof_15).op)) y →
      f₁ ((ConcreteCategory.hom (X.map (δ₂ 0 _proof_12 _proof_15).op)) x) =
          (ConcreteCategory.hom (Y.map (δ₂ 0 _proof_12 _proof_15).op)) y →
        f₁ ((ConcreteCategory.hom (X.map (δ₂ 1 _proof_12 _proof_15).op)) x) =
          (ConcreteCategory.hom (Y.map (δ₂ 1 _proof_12 _proof_15).op)) y
hσ :
  ∀ (x : X.obj (op { obj := ⦋0⦌, property := _proof_11 })),
    f₁ ((ConcreteCategory.hom (X.map (σ₂ 0 _proof_13 _proof_11).op)) x) =
      (ConcreteCategory.hom (Y.map (σ₂ 0 _proof_13 _proof_11).op)) (f₀ x)
hY : Y.StrictSegal
n : ℕ
hn : n < 2
i : Fin (n + 1)
⊢ naturalityProperty f₀ f₁ hδ₁ hδ₀ hY (σ 2 i ⋯ ⋯)

Tactic:
· obtain _ | _ | n := n
    · fin_cases i
      · ext; apply hδ₀
      · ext; apply hδ₁
    · fin_cases i
      · ext; apply hδ'₀ f₀ f₁ hδ₁ hδ₀ hY
      · ext; apply hδ'₁ f₀ f₁ hδ₁ hδ₀ H hY
      · ext; apply hδ'₂ f₀ f₁ hδ₁ hδ₀ hY
    · lia

Goal state after:
case refine_2
X Y : Truncated 2
f₀ : X.obj (op { obj := ⦋0⦌, property := _proof_11 }) → Y.obj (op { obj := ⦋0⦌, property := _proof_11 })
f₁ : X.obj (op { obj := ⦋1⦌, property := _proof_12 }) → Y.obj (op { obj := ⦋1⦌, property := _proof_12 })
hδ₁ :
  ∀ (x : X.obj (op { obj := ⦋1⦌, property := _proof_12 })),
    f₀ ((ConcreteCategory.hom (X.map (δ₂ 1 _proof_11 _proof_13).op)) x) =
      (ConcreteCategory.hom (Y.map (δ₂ 1 _proof_11 _proof_13).op)) (f₁ x)
hδ₀ :
  ∀ (x : X.obj (op { obj := ⦋1⦌, property := _proof_12 })),
    f₀ ((ConcreteCategory.hom (X.map (δ₂ 0 _proof_11 _proof_13).op)) x) =
      (ConcreteCategory.hom (Y.map (δ₂ 0 _proof_11 _proof_13).op)) (f₁ x)
H :
  ∀ (x : X.obj (op { obj := ⦋2⦌, property := _proof_14 })) (y : Y.obj (op { obj := ⦋2⦌, property := _proof_14 })),
    f₁ ((ConcreteCategory.hom (X.map (δ₂ 2 _proof_12 _proof_15).op)) x) =
        (ConcreteCategory.hom (Y.map (δ₂ 2 _proof_12 _proof_15).op)) y →
      f₁ ((ConcreteCategory.hom (X.map (δ₂ 0 _proof_12 _proof_15).op)) x) =
          (ConcreteCategory.hom (Y.map (δ₂ 0 _proof_12 _proof_15).op)) y →
        f₁ ((ConcreteCategory.hom (X.map (δ₂ 1 _proof_12 _proof_15).op)) x) =
          (ConcreteCategory.hom (Y.map (δ₂ 1 _proof_12 _proof_15).op)) y
hσ :
  ∀ (x : X.obj (op { obj := ⦋0⦌, property := _proof_11 })),
    f₁ ((ConcreteCategory.hom (X.map (σ₂ 0 _proof_13 _proof_11).op)) x) =
      (ConcreteCategory.hom (Y.map (σ₂ 0 _proof_13 _proof_11).op)) (f₀ x)
hY : Y.StrictSegal
n : ℕ
hn : n < 2
i : Fin (n + 1)
⊢ naturalityProperty f₀ f₁ hδ₁ hδ₀ hY (σ 2 i ⋯ ⋯)