-- Lean 4 proof step from Mathlib.CategoryTheory.Limits.FilteredColimitCommutesFiniteLimit (goal)
Goal:
J : Type u₁
K : Type u₂
inst : SmallCategory J
inst_1 : Category.{v₂, u₂} K
inst_2 : Small.{v, u₂} K
inst_3 : FinCategory J
F : J × K ⥤ Type v
inst_4 : IsFiltered.{v₂, u₂} K
x : limit (obj curry F ⋙ colim)
k : J → K
y : (j : J) → F.obj (j, k j)
e :
  ∀ (j : J),
    Eq (α := (fun (X : Type v) ↦ X) (colimit ((obj curry F).obj j)))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) (colimit.ι ((obj curry F).obj j) (k j)) :
            (a : (fun (X : Type v) ↦ X) (((curry.obj F).obj j).obj (k j))) →
              (fun (X : Type v) ↦ X) (colimit ((curry.obj F).obj j)))
          (y j) :
        (fun (X : Type v) ↦ X) (colimit ((obj curry F).obj j)))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) (limit.π (obj curry F ⋙ colim) j) :
            (a : (fun (X : Type v) ↦ X) (limit (curry.obj F ⋙ colim))) →
              (fun (X : Type v) ↦ X) ((curry.obj F ⋙ colim).obj j))
          x :
        (fun (X : Type v) ↦ X) ((obj curry F ⋙ colim).obj j))
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'),
    Eq (α := (fun (X : Type v) ↦ X) (F.obj (j', kf f)))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) (F.map (𝟙 j' ×ₘ g j' ≫ gf f)) :
            (a : (fun (X : Type v) ↦ X) (F.obj (j', k j'))) → (fun (X : Type v) ↦ X) (F.obj (j', kf f)))
          (y j') :
        (fun (X : Type v) ↦ X) (F.obj (j', kf f)))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) (F.map (f ×ₘ g j ≫ hf f)) :
            (a : (fun (X : Type v) ↦ X) (F.obj (j, k j))) → (fun (X : Type v) ↦ X) (F.obj (j', kf f)))
          (y j) :
        (fun (X : Type v) ↦ X) (F.obj (j', kf f)))
k'' : K
i : {j j' : J} → (f : j ⟶ j') → kf f ⟶ k''
s : ∀ {j₁ j₂ j₃ j₄ : J} (f : j₁ ⟶ j₂) (f' : j₃ ⟶ j₄), gf f ≫ i f = hf f' ≫ i f'
j j' : J
f : j ⟶ j'
⊢ Eq (α := F.obj (j', k''))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (F.map (f ×ₘ g j ≫ gf (𝟙 j) ≫ i (𝟙 j))) :
          (a : F.obj (j, k j)) → F.obj (j', k''))
        (y j) :
      F.obj (j', k''))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (F.map (𝟙 j' ×ₘ g j' ≫ gf (𝟙 j') ≫ i (𝟙 j'))) :
          (a : F.obj (j', k j')) → F.obj (j', k''))
        (y j') :
      F.obj (j', k''))
have j₁ : Eq (α := (fun (X : Type v) ↦ X) (F.obj (j', k'')))
  ((ConcreteCategory.hom (FC := TypeCat.Fun) (F.map (f ×ₘ g j ≫ gf (𝟙 j) ≫ i (𝟙 j))) :
        (a : (fun (X : Type v) ↦ X) (F.obj (j, k j))) → (fun (X : Type v) ↦ X) (F.obj (j', k'')))
      (y j) :
    (fun (X : Type v) ↦ X) (F.obj (j', k'')))
  ((ConcreteCategory.hom (FC := TypeCat.Fun) (F.map (f ×ₘ g j ≫ hf f ≫ i f)) :
        (a : (fun (X : Type v) ↦ X) (F.obj (j, k j))) → (fun (X : Type v) ↦ X) (F.obj (j', k'')))
      (y j) :
    (fun (X : Type v) ↦ X) (F.obj (j', k'')))
Tactic: calc
          F.map (f ×ₘ (g j ≫ gf (𝟙 j) ≫ i (𝟙 j))) (y j) =
              F.map (f ×ₘ (g j ≫ hf f ≫ i f)) (y j) := by
            rw [s (𝟙 j) f]
          _ =
              F.map (𝟙 j' ×ₘ i f) (F.map (f ×ₘ (g j ≫ hf f)) (y j)) := by
            rw [← comp_apply, ← Functor.map_comp, prod_comp, comp_id, assoc]
          _ =
              F.map (𝟙 j' ×ₘ i f) (F.map (𝟙 j' ×ₘ (g j' ≫ gf f)) (y j')) := by
            rw [← wf f]
          _ = F.map (𝟙 j' ×ₘ (g j' ≫ gf f ≫ i f)) (y j') := by
            rw [← comp_apply, ← Functor.map_comp, prod_comp, id_comp, assoc]
          _ = F.map (𝟙 j' ×ₘ (g j' ≫ gf (𝟙 j') ≫ i (𝟙 j'))) (y j') := by
            rw [s f (𝟙 j'), ← s (𝟙 j') (𝟙 j')]