-- 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
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 := ⋯ } } :
            { 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 := ⋯ } }))
        s) =
    s.ι.app j
have mk : ∀ (pt_1 : C)
  (ι :
    { obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C } ⋙ 𝟭 C ⟶
      Functor.obj (Functor.const J) pt_1),
  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 } }))
        { pt := pt_1, ι := ι }) =
    { pt := pt_1, ι := ι :
            Cocone
              ({ obj := obj, map := @map, map_id := map_id, map_comp := @map_comp : Functor.{w', v₁, w, u₁} J C } ⋙
                𝟭 C) }.ι.app
      j
Tactic: cases s
Cleared hypotheses: s