Module: Mathlib.CategoryTheory.Limits.VanKampen

Goal state before:
pp: case refine_3.succ.refine_1
C : Type u
inst✝² : Category.{v, u} C
n : ℕ
f : Fin (n + 1) → C
c₁ : Cofan fun i ↦ f i.succ
c₂ : BinaryCofan (f 0) c₁.pt
t₁ : IsVanKampenColimit c₁
t₂ : IsVanKampenColimit c₂
inst✝¹ : ∀ {Z : C} (i : Z ⟶ c₂.pt), HasPullback c₂.inr i
inst✝ : HasFiniteCoproducts C
F : Discrete (Fin (n + 1)) ⥤ C
c : Cocone F
α : F ⟶ Discrete.functor f
i : c.pt ⟶ (extendCofan c₁ c₂).pt
e : α ≫ (extendCofan c₁ c₂).ι = c.ι ≫ (const (Discrete (Fin (n + 1)))).map i
hα : NatTrans.Equifibered α
Hc : IsColimit c
t₂' :
  ∀ (j : Discrete WalkingPair),
    IsPullback ((BinaryCofan.mk (c.ι.app { as := 0 }) (Sigma.desc fun b ↦ c.ι.app { as := b.succ })).ι.app j)
      ((mapPair (α.app { as := 0 }) (Sigma.desc fun b ↦ α.app { as := b.succ } ≫ c₁.inj b)).app j) i (c₂.ι.app j)
j : Fin n
a✝ :
  IsPullback (c.ι.app { as := j.castSucc }) (α.app { as := j.castSucc }) i
    ((extendCofan c₁ c₂).ι.app { as := j.castSucc })
⊢ (Discrete.natTrans fun i ↦ α.app { as := i.as.succ }) ≫ c₁.ι =
    (Cofan.mk (colimit.cocone (Discrete.functor fun j ↦ F.obj { as := j.succ })).1
          (Sigma.ι fun j ↦ F.obj { as := j.succ })).ι ≫
      (const (Discrete (Fin n))).map (Sigma.desc fun j ↦ α.app { as := j.succ } ≫ c₁.inj j)

pp: case refine_3.succ.refine_2
C : Type u
inst✝² : Category.{v, u} C
n : ℕ
f : Fin (n + 1) → C
c₁ : Cofan fun i ↦ f i.succ
c₂ : BinaryCofan (f 0) c₁.pt
t₁ : IsVanKampenColimit c₁
t₂ : IsVanKampenColimit c₂
inst✝¹ : ∀ {Z : C} (i : Z ⟶ c₂.pt), HasPullback c₂.inr i
inst✝ : HasFiniteCoproducts C
F : Discrete (Fin (n + 1)) ⥤ C
c : Cocone F
α : F ⟶ Discrete.functor f
i : c.pt ⟶ (extendCofan c₁ c₂).pt
e : α ≫ (extendCofan c₁ c₂).ι = c.ι ≫ (const (Discrete (Fin (n + 1)))).map i
hα : NatTrans.Equifibered α
Hc : IsColimit c
t₂' :
  ∀ (j : Discrete WalkingPair),
    IsPullback ((BinaryCofan.mk (c.ι.app { as := 0 }) (Sigma.desc fun b ↦ c.ι.app { as := b.succ })).ι.app j)
      ((mapPair (α.app { as := 0 }) (Sigma.desc fun b ↦ α.app { as := b.succ } ≫ c₁.inj b)).app j) i (c₂.ι.app j)
j : Fin n
a✝ :
  IsPullback (c.ι.app { as := j.castSucc }) (α.app { as := j.castSucc }) i
    ((extendCofan c₁ c₂).ι.app { as := j.castSucc })
t₁' :
  IsPullback
    ((Cofan.mk (colimit.cocone (Discrete.functor fun j ↦ F.obj { as := j.succ })).1
            (Sigma.ι fun j ↦ F.obj { as := j.succ })).ι.app
      { as := j })
    ((Discrete.natTrans fun i ↦ α.app { as := i.as.succ }).app { as := j })
    (Sigma.desc fun j ↦ α.app { as := j.succ } ≫ c₁.inj j) (c₁.ι.app { as := j })
⊢ IsPullback (c.ι.app { as := j.succ }) (α.app { as := j.succ }) i ((extendCofan c₁ c₂).ι.app { as := j.succ })

Tactic:
· ext ⟨j⟩
      dsimp
      rw [colimit.ι_desc]
      rfl

Goal state after:
case refine_3.succ.refine_2
C : Type u
inst✝² : Category.{v, u} C
n : ℕ
f : Fin (n + 1) → C
c₁ : Cofan fun i ↦ f i.succ
c₂ : BinaryCofan (f 0) c₁.pt
t₁ : IsVanKampenColimit c₁
t₂ : IsVanKampenColimit c₂
inst✝¹ : ∀ {Z : C} (i : Z ⟶ c₂.pt), HasPullback c₂.inr i
inst✝ : HasFiniteCoproducts C
F : Discrete (Fin (n + 1)) ⥤ C
c : Cocone F
α : F ⟶ Discrete.functor f
i : c.pt ⟶ (extendCofan c₁ c₂).pt
e : α ≫ (extendCofan c₁ c₂).ι = c.ι ≫ (const (Discrete (Fin (n + 1)))).map i
hα : NatTrans.Equifibered α
Hc : IsColimit c
t₂' :
  ∀ (j : Discrete WalkingPair),
    IsPullback ((BinaryCofan.mk (c.ι.app { as := 0 }) (Sigma.desc fun b ↦ c.ι.app { as := b.succ })).ι.app j)
      ((mapPair (α.app { as := 0 }) (Sigma.desc fun b ↦ α.app { as := b.succ } ≫ c₁.inj b)).app j) i (c₂.ι.app j)
j : Fin n
a✝ :
  IsPullback (c.ι.app { as := j.castSucc }) (α.app { as := j.castSucc }) i
    ((extendCofan c₁ c₂).ι.app { as := j.castSucc })
t₁' :
  IsPullback
    ((Cofan.mk (colimit.cocone (Discrete.functor fun j ↦ F.obj { as := j.succ })).1
            (Sigma.ι fun j ↦ F.obj { as := j.succ })).ι.app
      { as := j })
    ((Discrete.natTrans fun i ↦ α.app { as := i.as.succ }).app { as := j })
    (Sigma.desc fun j ↦ α.app { as := j.succ } ≫ c₁.inj j) (c₁.ι.app { as := j })
⊢ IsPullback (c.ι.app { as := j.succ }) (α.app { as := j.succ }) i ((extendCofan c₁ c₂).ι.app { as := j.succ })