-- 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 : Cone { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C }
h :
  IsLimit (F :=
    { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C } ⋙ 𝟭 C)
    (Functor.mapCone (𝟭 C) c)
⊢ ∀ (s : Cone { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C })
    (j : J),
    (fun (s : Cone { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C }) ↦
            (h.lift { pt := s.pt, π := { app := fun (j : J) ↦ s.π.app j, naturality := ⋯ } } :
              Cone.pt.{w, v₁, w', u₁} (J := J) (F :=
                  { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C } ⋙
                    𝟭 C)
                  { pt := s.pt,
                    π :=
                      { app := fun (j : J) ↦ s.π.app j, naturality := ⋯ :
                        NatTrans (Functor.obj (Functor.const J) s.pt)
                          ({ obj := obj, map := @map, map_id := map_id, map_comp := @map_comp :
                              Functor.{w, v₁, w', u₁} J C } ⋙
                            𝟭 C) } :
                    Cone
                      ({ obj := obj, map := @map, map_id := map_id, map_comp := @map_comp :
                          Functor.{w, v₁, w', u₁} J C } ⋙
                        𝟭 C) } ⟶
                Cone.pt (F :=
                  { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C } ⋙
                    𝟭 C)
                  (Functor.mapCone (𝟭 C) c)))
          s ≫
        c.π.app j =
      s.π.app j
have mk : ∀ (pt : C)
  (app :
    (X : J) →
      Functor.obj.{w, v₁, w', u₁} (Functor.obj (Functor.const J) pt) X ⟶
        { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C }.obj X)
  (naturality :
    ∀ ⦃X Y : J⦄ (f : X ⟶ Y),
      Functor.map (Functor.obj (Functor.const J) pt) f ≫ app Y =
        app X ≫ { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp }.map f)
  (h :
    IsLimit (F :=
      { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C } ⋙ 𝟭 C)
      (Functor.mapCone (𝟭 C)
        { pt := pt,
          π :=
            { app := app, naturality := naturality :
              NatTrans (Functor.obj (Functor.const J) pt)
                { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C } } }))
  (s : Cone { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C }) (j : J),
  (fun (s : Cone { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C }) ↦
          (h.lift { pt := s.pt, π := { app := fun (j : J) ↦ s.π.app j, naturality := sorry } } :
            Cone.pt.{w, v₁, w', u₁} (J := J) (F :=
                { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C } ⋙
                  𝟭 C)
                { pt := s.pt,
                  π :=
                    { app := fun (j : J) ↦ s.π.app j, naturality := sorry :
                      NatTrans (Functor.obj (Functor.const J) s.pt)
                        ({ obj := obj, map := @map, map_id := map_id, map_comp := @map_comp :
                            Functor.{w, v₁, w', u₁} J C } ⋙
                          𝟭 C) } :
                  Cone
                    ({ obj := obj, map := @map, map_id := map_id, map_comp := @map_comp :
                        Functor.{w, v₁, w', u₁} J C } ⋙
                      𝟭 C) } ⟶
              Cone.pt (F :=
                { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w, v₁, w', u₁} J C } ⋙
                  𝟭 C)
                (Functor.mapCone (𝟭 C)
                  { pt := pt,
                    π :=
                      { app := app, naturality := naturality :
                        NatTrans (Functor.obj (Functor.const J) pt)
                          { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp :
                            Functor.{w, v₁, w', u₁} J C } } })))
        s ≫
      { pt := pt,
              π :=
                { app := app, naturality := naturality :
                  NatTrans (Functor.obj (Functor.const J) pt)
                    { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp :
                      Functor.{w, v₁, w', u₁} J C } } :
              Cone
                { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp :
                  Functor.{w, v₁, w', u₁} J C } }.π.app
        j =
    s.π.app j
Tactic: rcases c with ⟨_, _, _⟩
Cleared hypotheses: c, h