theorem isUniversalColimit_extendCofan {n : ℕ} (f : Fin (n + 1) → C)
    {c₁ : Cofan fun i : Fin n ↦ f i.succ} {c₂ : BinaryCofan (f 0) c₁.pt}
    (t₁ : IsUniversalColimit c₁) (t₂ : IsUniversalColimit c₂)
    [∀ {Z} (i : Z ⟶ c₂.pt), HasPullback c₂.inr i] :
    IsUniversalColimit (extendCofan c₁ c₂) := by
  intro F c α i e hα H
  let F' : Fin (n + 1) → C := F.obj ∘ Discrete.mk
  have : F = Discrete.functor F'
  · apply Functor.hext
    · exact fun i ↦ rfl
    · rintro ⟨i⟩ ⟨j⟩ ⟨⟨rfl : i = j⟩⟩
      simp
  have t₁' := @t₁ (Discrete.functor (fun j ↦ F.obj ⟨j.succ⟩))
    (Cofan.mk (pullback c₂.inr i) <| fun j ↦ pullback.lift (α.app _ ≫ c₁.inj _) (c.ι.app _) ?_)
    (Discrete.natTrans <| fun i ↦ α.app _) pullback.fst ?_ (NatTrans.equifibered_of_discrete _) ?_
  rotate_left
  · simpa only [Functor.const_obj_obj, pair_obj_right, Discrete.functor_obj, Category.assoc,
      extendCofan_pt, Functor.const_obj_obj, NatTrans.comp_app, extendCofan_ι_app,
      Fin.cases_succ, Functor.const_map_app] using congr_app e ⟨j.succ⟩
  · ext j
    dsimp
    simp only [limit.lift_π, PullbackCone.mk_pt, PullbackCone.mk_π_app, Cofan.inj]
  · intro j
    simp only [pair_obj_right, Functor.const_obj_obj, Discrete.functor_obj, id_eq,
      extendCofan_pt, eq_mpr_eq_cast, Cofan.mk_pt, Cofan.mk_ι_app, Discrete.natTrans_app]
    refine IsPullback.of_right ?_ ?_ (IsPullback.of_hasPullback (BinaryCofan.inr c₂) i).flip
    · simp only [Functor.const_obj_obj, pair_obj_right, limit.lift_π,
        PullbackCone.mk_pt, PullbackCone.mk_π_app]
      exact H _
    · simp only [limit.lift_π, PullbackCone.mk_pt, PullbackCone.mk_π_app, Cofan.inj]
  obtain ⟨H₁⟩ := t₁'
  have t₂' := @t₂ (pair (F.obj ⟨0⟩) (pullback c₂.inr i)) (BinaryCofan.mk (c.ι.app ⟨0⟩) pullback.snd)
    (mapPair (α.app _) pullback.fst) i ?_ (mapPair_equifibered _) ?_
  rotate_left
  · ext ⟨⟨⟩⟩
    · simpa [mapPair] using congr_app e ⟨0⟩
    · simpa using pullback.condition
  · rintro ⟨⟨⟩⟩
    · simp only [pair_obj_right, Functor.const_obj_obj, pair_obj_left, BinaryCofan.mk_pt,
        BinaryCofan.ι_app_left, BinaryCofan.mk_inl, mapPair_left]
      exact H ⟨0⟩
    · simp only [pair_obj_right, Functor.const_obj_obj, BinaryCofan.mk_pt, BinaryCofan.ι_app_right,
        BinaryCofan.mk_inr, mapPair_right]
      exact (IsPullback.of_hasPullback (BinaryCofan.inr c₂) i).flip
  

Proof state:
case refine_4.intro.refine_3
J : Type v'
inst✝⁴ : Category.{u', v'} J
C : Type u
inst✝³ : Category.{v, u} C
K : Type u_1
inst✝² : Category.{?u.298062, u_1} K
D : Type u_2
inst✝¹ : Category.{?u.298069, u_2} D
n : ℕ
f : Fin (n + 1) → C
c₁ : Cofan fun i => f (Fin.succ i)
c₂ : BinaryCofan (f 0) c₁.pt
t₁ : IsUniversalColimit c₁
t₂ : IsUniversalColimit c₂
inst✝ : ∀ {Z : C} (i : Z ⟶ c₂.pt), HasPullback (BinaryCofan.inr c₂) i
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 ≫ (Functor.const (Discrete (Fin (n + 1)))).map i
hα : NatTrans.Equifibered α
H : ∀ (j : Discrete (Fin (n + 1))), IsPullback (c.ι.app j) (α.app j) i ((extendCofan c₁ c₂).ι.app j)
F' : Fin (n + 1) → C := F.obj ∘ Discrete.mk
this : F = Discrete.functor F'
H₁ :
  IsColimit
    (Cofan.mk (pullback (BinaryCofan.inr c₂) i) fun j =>
      pullback.lift (α.app { as := Fin.succ j } ≫ Cofan.inj c₁ j) (c.ι.app { as := Fin.succ j })
        (_ : (α.app { as := Fin.succ j } ≫ Cofan.inj c₁ j) ≫ BinaryCofan.inr c₂ = c.ι.app { as := Fin.succ j } ≫ i))
t₂' : Nonempty (IsColimit (BinaryCofan.mk (c.ι.app { as := 0 }) pullback.snd))
⊢ Nonempty (IsColimit c)

Next tactic:
obtain ⟨H₂⟩ := t₂'