-- Lean 4 proof step from Mathlib.CategoryTheory.Limits.Preserves.Basic (goal)
Goal:
C : Type u₁
inst : Category.{v₁, u₁} C
D : Type u₂
inst_1 : Category.{v₂, u₂} D
J_1 : Type w
inst_2 : Category.{w', w} J_1
K : Functor.{w', v₁, w, u₁} J_1 C
J : Type w
𝒥 : Category.{w', w} J
obj : J → C
map : {X Y : J} → (X ⟶ Y) → (obj X ⟶ obj Y)
map_id : ∀ (X : J), map (𝟙 X) = 𝟙 (obj X)
map_comp : ∀ {X Y Z : J} (f : X ⟶ Y) (g : Y ⟶ Z), map (f ≫ g) = map f ≫ map g
c : Cocone { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C }
h : IsColimit c
⊢ ∀
    (s :
      Cocone ({ obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C } ⋙ 𝟭 C))
    (j : J),
    CategoryStruct.comp (Y :=
        Functor.obj.{w', v₁, w, u₁}
          (Functor.obj (Functor.const J)
            (Cocone.pt (F :=
              { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C } ⋙ 𝟭 C)
              (Functor.mapCocone (𝟭 C) c)))
          j)
        (NatTrans.app (Cocone.ι (Functor.mapCocone (𝟭 C) c)) j)
        ((fun
              (s :
                Cocone
                  ({ obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C } ⋙
                    𝟭 C)) ↦
            (h.desc { pt := s.pt, ι := { app := fun (j : J) ↦ s.ι.app j, naturality := ⋯ } } :
              c.pt ⟶
                Cocone.pt.{w', v₁, w, u₁} (J := J) (F :=
                  { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp })
                  { pt := s.pt, ι := { app := fun (j : J) ↦ s.ι.app j, naturality := ⋯ } }))
          s) =
      s.ι.app j
have mk : ∀ (pt : C)
  (app :
    (X : J) →
      { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C }.obj X ⟶
        Functor.obj.{w', v₁, w, u₁} (Functor.obj (Functor.const J) pt) X)
  (naturality :
    ∀ ⦃X Y : J⦄ (f : X ⟶ Y),
      { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C }.map f ≫ app Y =
        app X ≫ Functor.map (Functor.obj (Functor.const J) pt) f)
  (h :
    IsColimit
      { pt := pt,
        ι :=
          { app := app, naturality := naturality :
            NatTrans { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C }
              (Functor.obj (Functor.const J) pt) } :
        Cocone { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C } })
  (s :
    Cocone ({ obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C } ⋙ 𝟭 C))
  (j : J),
  CategoryStruct.comp (Y :=
      Functor.obj.{w', v₁, w, u₁}
        (Functor.obj (Functor.const J)
          (Cocone.pt (F :=
            { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C } ⋙ 𝟭 C)
            (Functor.mapCocone (𝟭 C)
              { pt := pt,
                ι :=
                  { app := app, naturality := naturality :
                    NatTrans
                      { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C }
                      (Functor.obj (Functor.const J) pt) } :
                Cocone
                  { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp :
                    Functor.{w', v₁, w, u₁} J C } })))
        j)
      (NatTrans.app
        (Cocone.ι
          (Functor.mapCocone (𝟭 C)
            { pt := pt,
              ι :=
                { app := app, naturality := naturality :
                  NatTrans
                    { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C }
                    (Functor.obj (Functor.const J) pt) } }))
        j)
      ((fun
            (s :
              Cocone
                ({ obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C } ⋙
                  𝟭 C)) ↦
          (h.desc { pt := s.pt, ι := { app := fun (j : J) ↦ s.ι.app j, naturality := sorry } } :
            { pt := pt,
                  ι :=
                    { app := app, naturality := naturality :
                      NatTrans
                        { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp :
                          Functor.{w', v₁, w, u₁} J C }
                        (Functor.obj (Functor.const J) pt) } :
                  Cocone
                    { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp :
                      Functor.{w', v₁, w, u₁} J C } }.pt ⟶
              Cocone.pt.{w', v₁, w, u₁} (J := J) (F :=
                { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp })
                { pt := s.pt, ι := { app := fun (j : J) ↦ s.ι.app j, naturality := sorry } }))
        s) =
    s.ι.app j
Tactic: rcases c with ⟨_, _, _⟩
Cleared hypotheses: c, h