Module: Mathlib.CategoryTheory.Limits.FilteredColimitCommutesFiniteLimit

Goal state before:
pp: J : Type u₁
K : Type u₂
inst✝⁴ : SmallCategory J
inst✝³ : Category.{v₂, u₂} K
inst✝² : Small.{v, u₂} K
inst✝¹ : FinCategory J
F : J × K ⥤ Type v
inst✝ : IsFiltered K
x : limit (curry.obj F ⋙ colim)
k : J → K
y : (j : J) → F.obj (j, k j)
e :
  ∀ (j : J),
    (ConcreteCategory.hom (colimit.ι ((curry.obj F).obj j) (k j))) (y j) =
      (ConcreteCategory.hom (limit.π (curry.obj F ⋙ colim) j)) x
k' : K
g : (j : J) → k j ⟶ k'
kf : {j j' : J} → (j ⟶ j') → K
gf hf : {j j' : J} → (f : j ⟶ j') → k' ⟶ kf f
wf :
  ∀ {j j' : J} (f : j ⟶ j'),
    (ConcreteCategory.hom (F.map (𝟙 j' ×ₘ g j' ≫ gf f))) (y j') = (ConcreteCategory.hom (F.map (f ×ₘ g j ≫ hf f))) (y j)
O : Finset K := (Finset.univ.biUnion fun j ↦ Finset.univ.biUnion fun j' ↦ Finset.image kf Finset.univ) ∪ {k'}
kfO : ∀ {j j' : J} (f : j ⟶ j'), kf f ∈ O
k'O : k' ∈ O
H : Finset ((X : K) ×' (Y : K) ×' (_ : X ∈ O) ×' (_ : Y ∈ O) ×' (X ⟶ Y)) :=
  Finset.univ.biUnion fun j ↦
    Finset.univ.biUnion fun j' ↦
      Finset.univ.biUnion fun f ↦ {⟨k', ⟨kf f, ⟨k'O, ⟨⋯, gf f⟩⟩⟩⟩, ⟨k', ⟨kf f, ⟨k'O, ⟨⋯, hf f⟩⟩⟩⟩}
k'' : K
i' : {X : K} → X ∈ O → (X ⟶ k'')
s' : ∀ {X Y : K} (mX : X ∈ O) (mY : Y ∈ O) {f : X ⟶ Y}, ⟨X, ⟨Y, ⟨mX, ⟨mY, f⟩⟩⟩⟩ ∈ H → f ≫ i' mY = i' mX
i : {j j' : J} → (f : j ⟶ j') → kf f ⟶ k'' := fun {j j'} f ↦ i' ⋯
⊢ ∃ a, (ConcreteCategory.hom (colimitLimitToLimitColimit F)) a = x

Tactic:
have s : ∀ {j₁ j₂ j₃ j₄} (f : j₁ ⟶ j₂) (f' : j₃ ⟶ j₄), gf f ≫ i f = hf f' ≫ i f' := by
      intro j₁ j₂ j₃ j₄ f f'
      rw [s', s']
      · exact k'O
      · exact Finset.mem_biUnion.mpr ⟨j₃, Finset.mem_univ _,
          Finset.mem_biUnion.mpr ⟨j₄, Finset.mem_univ _,
            Finset.mem_biUnion.mpr ⟨f', Finset.mem_univ _, by
              -- This works by `simp`, but has very high variation in heartbeats.
              rw [Finset.mem_insert, PSigma.mk.injEq, heq_eq_eq, PSigma.mk.injEq, heq_eq_eq,
                PSigma.mk.injEq, heq_eq_eq, PSigma.mk.injEq, heq_eq_eq, eq_self, true_and, eq_self,
                true_and, eq_self, true_and, eq_self, true_and, Finset.mem_singleton, eq_self,
                or_true]
              trivial⟩⟩⟩
      · exact Finset.mem_biUnion.mpr ⟨j₁, Finset.mem_univ _,
          Finset.mem_biUnion.mpr ⟨j₂, Finset.mem_univ _,
            Finset.mem_biUnion.mpr ⟨f, Finset.mem_univ _, by
              -- This works by `simp`, but has very high variation in heartbeats.
              rw [Finset.mem_insert, PSigma.mk.injEq, heq_eq_eq, PSigma.mk.injEq, heq_eq_eq,
                PSigma.mk.injEq, heq_eq_eq, PSigma.mk.injEq, heq_eq_eq, eq_self, true_and, eq_self,
                true_and, eq_self, true_and, eq_self, true_and, Finset.mem_singleton, eq_self,
                true_or]
              trivial⟩⟩⟩

Goal state after:
J : Type u₁
K : Type u₂
inst✝⁴ : SmallCategory J
inst✝³ : Category.{v₂, u₂} K
inst✝² : Small.{v, u₂} K
inst✝¹ : FinCategory J
F : J × K ⥤ Type v
inst✝ : IsFiltered K
x : limit (curry.obj F ⋙ colim)
k : J → K
y : (j : J) → F.obj (j, k j)
e :
  ∀ (j : J),
    (ConcreteCategory.hom (colimit.ι ((curry.obj F).obj j) (k j))) (y j) =
      (ConcreteCategory.hom (limit.π (curry.obj F ⋙ colim) j)) x
k' : K
g : (j : J) → k j ⟶ k'
kf : {j j' : J} → (j ⟶ j') → K
gf hf : {j j' : J} → (f : j ⟶ j') → k' ⟶ kf f
wf :
  ∀ {j j' : J} (f : j ⟶ j'),
    (ConcreteCategory.hom (F.map (𝟙 j' ×ₘ g j' ≫ gf f))) (y j') = (ConcreteCategory.hom (F.map (f ×ₘ g j ≫ hf f))) (y j)
O : Finset K := (Finset.univ.biUnion fun j ↦ Finset.univ.biUnion fun j' ↦ Finset.image kf Finset.univ) ∪ {k'}
kfO : ∀ {j j' : J} (f : j ⟶ j'), kf f ∈ O
k'O : k' ∈ O
H : Finset ((X : K) ×' (Y : K) ×' (_ : X ∈ O) ×' (_ : Y ∈ O) ×' (X ⟶ Y)) :=
  Finset.univ.biUnion fun j ↦
    Finset.univ.biUnion fun j' ↦
      Finset.univ.biUnion fun f ↦ {⟨k', ⟨kf f, ⟨k'O, ⟨⋯, gf f⟩⟩⟩⟩, ⟨k', ⟨kf f, ⟨k'O, ⟨⋯, hf f⟩⟩⟩⟩}
k'' : K
i' : {X : K} → X ∈ O → (X ⟶ k'')
s' : ∀ {X Y : K} (mX : X ∈ O) (mY : Y ∈ O) {f : X ⟶ Y}, ⟨X, ⟨Y, ⟨mX, ⟨mY, f⟩⟩⟩⟩ ∈ H → f ≫ i' mY = i' mX
i : {j j' : J} → (f : j ⟶ j') → kf f ⟶ k'' := fun {j j'} f ↦ i' ⋯
s : ∀ {j₁ j₂ j₃ j₄ : J} (f : j₁ ⟶ j₂) (f' : j₃ ⟶ j₄), gf f ≫ i f = hf f' ≫ i f'
⊢ ∃ a, (ConcreteCategory.hom (colimitLimitToLimitColimit F)) a = x