-- 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)
x : c.pt.obj cX.pt
this : IsFiltered J
y : (↑(F ⋙ obj (evaluation C (Type w')) cX.pt).sections : Type (max w' u'))
hy :
  (Equiv.symm (Types.isLimitEquivSections (hc cX.pt)) :
        (a : (↑(F ⋙ (evaluation C (Type w')).obj cX.pt).sections : Type (max w' u'))) →
          (((evaluation C (Type w')).obj cX.pt).mapCone c).pt)
      y =
    x
j₀ : J
z : (k : K) → (F.obj k).obj (X.obj j₀)
hz :
  ∀ (k : K),
    (↑y : (j : K) → (F ⋙ (evaluation C (Type w')).obj cX.pt).obj j) k =
      (ConcreteCategory.hom (FC := TypeCat.Fun) ((F.obj k).map (cX.ι.app j₀)) :
          (a : (fun (X : Type w') ↦ X) ((F.obj k).obj (X.obj j₀))) →
            (fun (X : Type w') ↦ X) ((F.obj k).obj (((const J).obj cX.pt).obj j₀)))
        (z k)
j₁ : J
α : j₀ ⟶ j₁
hα :
  ∀ ⦃k k' : K⦄ (φ : k ⟶ k'),
    Eq (α := (fun (X : Type w') ↦ X) ((F.obj k').obj (X.obj j₁)))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) ((F.obj k').map (X.map α)) :
            (a : (fun (X : Type w') ↦ X) ((F.obj k').obj (X.obj j₀))) →
              (fun (X : Type w') ↦ X) ((F.obj k').obj (X.obj j₁)))
          ((ConcreteCategory.hom (FC := TypeCat.Fun) ((F.map φ).app (X.obj j₀)) :
                (a : (fun (X : Type w') ↦ X) ((F.obj k).obj (X.obj j₀))) →
                  (fun (X : Type w') ↦ X) ((F.obj k').obj (X.obj j₀)))
              (z k) :
            (fun (X : Type w') ↦ X) ((F.obj k').obj (X.obj j₀))) :
        (fun (X : Type w') ↦ X) ((F.obj k').obj (X.obj j₁)))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) ((F.obj k').map (X.map α)) :
            (a : (fun (X : Type w') ↦ X) ((F.obj k').obj (X.obj j₀))) →
              (fun (X : Type w') ↦ X) ((F.obj k').obj (X.obj j₁)))
          (z k') :
        (fun (X : Type w') ↦ X) ((F.obj k').obj (X.obj j₁)))
s : (↑(F ⋙ obj (evaluation C (Type w')) (X.obj j₁)).sections : Type (max u' w')) :=
  Subtype.mk (p := fun (x : (j : K) → (F ⋙ obj (evaluation C (Type w')) (X.obj j₁)).obj j) ↦
    x ∈ (F ⋙ obj (evaluation C (Type w')) (X.obj j₁)).sections)
    (fun (k : K) ↦
      (ConcreteCategory.hom (FC := TypeCat.Fun) ((F.obj k).map (X.map α)) :
          (a : (fun (X : Type w') ↦ X) ((F.obj k).obj (X.obj j₀))) → (fun (X : Type w') ↦ X) ((F.obj k).obj (X.obj j₁)))
        (z k))
    ⋯
⊢ ∃ (j : J) (x' : c.pt.obj (X.obj j)),
    x =
      (ConcreteCategory.hom (FC := TypeCat.Fun) ((c.pt.mapCocone cX).ι.app j) :
          (a : (fun (X : Type w') ↦ X) ((X ⋙ c.pt).obj j)) →
            (fun (X : Type w') ↦ X) (((const J).obj (c.pt.mapCocone cX).pt).obj j))
        x'
have refine : x =
  (ConcreteCategory.hom (FC := TypeCat.Fun) ((c.pt.mapCocone cX).ι.app j₁) :
      (a : (fun (X : Type w') ↦ X) ((X ⋙ c.pt).obj j₁)) →
        (fun (X : Type w') ↦ X) (((const J).obj (c.pt.mapCocone cX).pt).obj j₁))
    ((Equiv.symm (Types.isLimitEquivSections (hc (X.obj j₁))) :
          (a : (↑(F ⋙ (evaluation C (Type w')).obj (X.obj j₁)).sections : Type (max w' u'))) →
            (((evaluation C (Type w')).obj (X.obj j₁)).mapCone c).pt)
        s :
      Cone.pt (F := F ⋙ obj (evaluation C (Type w')) (X.obj j₁)) (mapCone (obj (evaluation C (Type w')) (X.obj j₁)) c))
Tactic: refine ⟨j₁, (Types.isLimitEquivSections (hc (X.obj j₁))).symm s, ?_⟩