-- Lean 4 proof step from Mathlib.CategoryTheory.Extensive (goal)
Goal:
J : Type v'
inst : Category.{u', v'} J
C : Type u
inst_1 : Category.{v, u} C
D : Type u''
inst_2 : Category.{v'', u''} D
X Y : C
⊢ FinitaryExtensive (Type u)
have rw : IsVanKampenColimit (Types.binaryCoproductCocone PUnit.{u + 1} PUnit.{u + 1})
Tactic: classical
  rw [finitaryExtensive_iff_of_isTerminal (Type u) PUnit Types.isTerminalPUnit _
      (Types.binaryCoproductColimit _ _)]
  apply BinaryCofan.isVanKampen_mk _ _ (fun X Y => Types.binaryCoproductColimit X Y) _
      fun f g => (Limits.Types.pullbackLimitCone f g).2
  · intro _ _ _ _ f hαX hαY
    constructor
    · refine ⟨⟨hαX.symm⟩, ⟨PullbackCone.isLimitAux' _ ?_⟩⟩
      intro s
      have : ∀ x, ∃! y, s.fst x = Sum.inl y := by
        intro x
        rcases h : s.fst x with val | val
        · simp
        · apply_fun f at h
          cases ((ConcreteCategory.congr_hom s.condition x).symm.trans h).trans
            (ConcreteCategory.congr_hom hαY val :).symm
      delta ExistsUnique at this
      choose l hl hl' using this
      refine ⟨↾(l), ?_, Types.isTerminalPUnit.hom_ext _ _, fun {l'} h₁ _ => ?_⟩
      · ext x
        exact (hl x).symm
      · ext x
        exact hl' x (l' x) (ConcreteCategory.congr_hom h₁ x).symm
    · refine ⟨⟨hαY.symm⟩, ⟨PullbackCone.isLimitAux' _ ?_⟩⟩
      intro s
      have : ∀ x, ∃! y, s.fst x = Sum.inr y := by
        intro x
        rcases h : s.fst x with val | val
        · apply_fun f at h
          cases ((ConcreteCategory.congr_hom s.condition x).symm.trans h).trans
            (ConcreteCategory.congr_hom hαX val :).symm
        · simp
      delta ExistsUnique at this
      choose l hl hl' using this
      refine ⟨↾l, ?_, Types.isTerminalPUnit.hom_ext _ _, fun {l'} h₁ _ => ?_⟩
      · ext x
        exact (hl x).symm
      · ext x
        exact hl' x (l' x) (ConcreteCategory.congr_hom h₁ x).symm
  · intro Z f
    dsimp [Limits.Types.binaryCoproductCocone]
    have : ∀ x, f x = Sum.inl PUnit.unit ∨ f x = Sum.inr PUnit.unit := by
      intro x
      rcases f x with (⟨⟨⟩⟩ | ⟨⟨⟩⟩)
      exacts [Or.inl rfl, Or.inr rfl]
    let eX : { p : Z × PUnit // f p.fst = Sum.inl p.snd } ≃ { x : Z // f x = Sum.inl PUnit.unit } :=
      ⟨fun p => ⟨p.1.1, by convert! p.2⟩, fun x => ⟨⟨_, _⟩, x.2⟩, fun _ => by ext; rfl,
        fun _ => by ext; rfl⟩
    let eY : { p : Z × PUnit // f p.fst = Sum.inr p.snd } ≃ { x : Z // f x = Sum.inr PUnit.unit } :=
      ⟨fun p => ⟨p.1.1, p.2.trans (congr_arg Sum.inr <| Subsingleton.elim _ _)⟩,
        fun x => ⟨⟨_, _⟩, x.2⟩, fun _ => by ext; rfl, fun _ => by ext; rfl⟩
    fapply BinaryCofan.isColimitMk
    · exact fun s => ↾fun x => dite _ (fun h => s.inl <| eX.symm ⟨x, h⟩)
        fun h => s.inr <| eY.symm ⟨x, (this x).resolve_left h⟩
    · intro s
      ext ⟨⟨x, ⟨⟩⟩, _⟩
      dsimp
      split_ifs with h <;> tauto
    · intro s
      ext ⟨⟨x, ⟨⟩⟩, hx⟩
      dsimp
      split_ifs with h
      · cases h.symm.trans hx
      · rfl
    · intro s m e₁ e₂
      ext x
      simp only [TypeCat.Fun.toFun_apply, Types.binaryCoproductCocone_pt, pair_obj_left,
        Functor.const_obj_obj, pair_obj_right, ConcreteCategory.hom_ofHom, TypeCat.Fun.coe_mk]
      split_ifs
      · rw [← e₁]
        rfl
      · rw [← e₂]
        rfl