-- Lean 4 proof step from Mathlib.CategoryTheory.Presentable.Limits (goal)
Goal:
C : Type u
inst : Category.{v, u} C
K : Type u'
inst_1 : Category.{v', u'} K
F : K ⥤ Functor.{v, w', u, w' + 1} C (Type w')
c : Cone F
hc : (Y : C) → IsLimit (F := F ⋙ obj (evaluation C (Type w')) Y) (mapCone (obj (evaluation C (Type w')) Y) c)
κ : Cardinal.{w}
inst_2 : Fact κ.IsRegular
hK : HasCardinalLT (Arrow.{v', u'} K) κ
J : Type w
inst_3 : SmallCategory J
inst_4 : IsCardinalFiltered J κ
X : Functor.{w, v, w, u} J C
cX : Cocone X
hF : (k : K) → IsColimit ((F.obj k).mapCocone cX)
j : J
x₁ x₂ : c.pt.obj (X.obj j)
h :
  Eq (α := (fun (X : Type w') ↦ X) (c.pt.obj (obj.{w, v, w, u} (obj (const J) cX.pt) j)))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (c.pt.map (cX.ι.app j)) :
          (a : (fun (X : Type w') ↦ X) (c.pt.obj (X.obj j))) →
            (fun (X : Type w') ↦ X) (c.pt.obj (((const J).obj cX.pt).obj j)))
        x₁ :
      (fun (X : Type w') ↦ X) (c.pt.obj (obj.{w, v, w, u} (obj (const J) cX.pt) j)))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (c.pt.map (cX.ι.app j)) :
          (a : (fun (X : Type w') ↦ X) (c.pt.obj (X.obj j))) →
            (fun (X : Type w') ↦ X) (c.pt.obj (((const J).obj cX.pt).obj j)))
        x₂ :
      (fun (X : Type w') ↦ X) (c.pt.obj (obj.{w, v, w, u} (obj (const J) cX.pt) j)))
this : IsFiltered J
y₁ : (↑(F ⋙ obj (evaluation C (Type w')) (X.obj j)).sections : Type (max w' u')) :=
  (Types.isLimitEquivSections (hc (X.obj j)) :
      (a : (((evaluation C (Type w')).obj (X.obj j)).mapCone c).pt) →
        (↑(F ⋙ (evaluation C (Type w')).obj (X.obj j)).sections : Type (max w' u')))
    x₁
y₂ : (↑(F ⋙ obj (evaluation C (Type w')) (X.obj j)).sections : Type (max w' u')) :=
  (Types.isLimitEquivSections (hc (X.obj j)) :
      (a : (((evaluation C (Type w')).obj (X.obj j)).mapCone c).pt) →
        (↑(F ⋙ (evaluation C (Type w')).obj (X.obj j)).sections : Type (max w' u')))
    x₂
hy₁ :
  (Equiv.symm (Types.isLimitEquivSections (hc (X.obj j))) :
        (a : (↑(F ⋙ (evaluation C (Type w')).obj (X.obj j)).sections : Type (max w' u'))) →
          (((evaluation C (Type w')).obj (X.obj j)).mapCone c).pt)
      y₁ =
    x₁
hy₂ :
  (Equiv.symm (Types.isLimitEquivSections (hc (X.obj j))) :
        (a : (↑(F ⋙ (evaluation C (Type w')).obj (X.obj j)).sections : Type (max w' u'))) →
          (((evaluation C (Type w')).obj (X.obj j)).mapCone c).pt)
      y₂ =
    x₂
k : K
⊢ Eq (α := obj (obj (const J) ((F.obj k).mapCocone cX).pt) j)
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (((F.obj k).mapCocone cX).ι.app j) :
          (a : (X ⋙ F.obj k).obj j) → ((const J).obj ((F.obj k).mapCocone cX).pt).obj j)
        ((ConcreteCategory.hom (FC := TypeCat.Fun)
                (NatTrans.app (Cone.π (mapCone (obj (evaluation C (Type w')) (X.obj j)) c)) k) :
              (a : ((const K).obj (((evaluation C (Type w')).obj (X.obj j)).mapCone c).pt).obj k) →
                (F ⋙ (evaluation C (Type w')).obj (X.obj j)).obj k)
            x₁ :
          (F ⋙ obj (evaluation C (Type w')) (X.obj j)).obj k) :
      obj (obj (const J) ((F.obj k).mapCocone cX).pt) j)
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (((F.obj k).mapCocone cX).ι.app j) :
          (a : (X ⋙ F.obj k).obj j) → ((const J).obj ((F.obj k).mapCocone cX).pt).obj j)
        ((ConcreteCategory.hom (FC := TypeCat.Fun)
                (NatTrans.app (Cone.π (mapCone (obj (evaluation C (Type w')) (X.obj j)) c)) k) :
              (a : ((const K).obj (((evaluation C (Type w')).obj (X.obj j)).mapCone c).pt).obj k) →
                (F ⋙ (evaluation C (Type w')).obj (X.obj j)).obj k)
            x₂ :
          (F ⋙ obj (evaluation C (Type w')) (X.obj j)).obj k) :
      obj (obj (const J) ((F.obj k).mapCocone cX).pt) j)
have dsimp : Eq (α := (F.obj k).obj cX.pt)
  ((ConcreteCategory.hom (FC := TypeCat.Fun) ((F.obj k).map (cX.ι.app j)) :
        (a : (F.obj k).obj (X.obj j)) → (F.obj k).obj cX.pt)
      ((ConcreteCategory.hom (FC := TypeCat.Fun) ((c.π.app k).app (X.obj j)) :
            (a : c.pt.obj (X.obj j)) → (F.obj k).obj (X.obj j))
          x₁ :
        (F.obj k).obj (X.obj j)) :
    (F.obj k).obj cX.pt)
  ((ConcreteCategory.hom (FC := TypeCat.Fun) ((F.obj k).map (cX.ι.app j)) :
        (a : (F.obj k).obj (X.obj j)) → (F.obj k).obj cX.pt)
      ((ConcreteCategory.hom (FC := TypeCat.Fun) ((c.π.app k).app (X.obj j)) :
            (a : c.pt.obj (X.obj j)) → (F.obj k).obj (X.obj j))
          x₂ :
        (F.obj k).obj (X.obj j)) :
    (F.obj k).obj cX.pt)
Tactic: dsimp at h ⊢