-- Lean 4 proof step from Mathlib.Condensed.Light.Sequence (goal)
Goal:
R : Type
inst : CommRing R
X Y : LightCondMod R
p : X ⟶ Y
hp : Epi p
S : LightProfinite
f :
  obj (free R)
      (tensorObj (𝒞 :=
          CompHausLike.category fun (X : TopCat.{0}) ↦
            TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))
          S ℕ∪{∞}).toCondensed ⟶
    Y
T : LightProfinite
π :
  T ⟶
    tensorObj (𝒞 :=
      CompHausLike.category fun (X : TopCat.{0}) ↦
        TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))
      S ℕ∪{∞}
g : obj (free R) T.toCondensed ⟶ X
hπ : Epi π
comm : CategoryTheory.Functor.map (lightProfiniteToLightCondSet ⋙ free R) π ≫ f = g ≫ p
⊢ ∃ (S' : LightProfinite.{0}) (π : S' ⟶ S) (_ :
    Function.Surjective (α :=
      (fun
            (X :
              CompHausLike fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) ↦
          (↑(X.toTop (P := fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
            Type))
        S')
      (β :=
      (fun
            (X :
              CompHausLike fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) ↦
          (↑(X.toTop (P := fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
            Type))
        S)
      (⇑(ConcreteCategory.hom (FC :=
            fun
              (x1 x2 :
                CompHausLike fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) ↦
            C((↑(x1.toTop (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
                Type),
              (↑(x2.toTop (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
                Type)))
            (CC :=
            fun
              (X :
                CompHausLike fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) ↦
            (↑(X.toTop (P := fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
              Type))
            π) :
        (a :
            (fun
                  (X :
                    CompHausLike fun (X : TopCat) ↦
                      TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) ↦
                (↑X.toTop : Type))
              S') →
          (fun
                (X :
                  CompHausLike fun (X : TopCat) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) ↦
              (↑X.toTop : Type))
            S))
    (g' :
    obj (free R)
        (tensorObj (𝒞 :=
            CompHausLike.category fun (X : TopCat.{0}) ↦
              TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))
            S' ℕ∪{∞}).toCondensed ⟶
      X),
    CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map (π ▷ ℕ∪{∞})) ≫ f = g' ≫ p
have obtain : ∀ (S' T' : LightProfinite.{0}) (y' : S' ⟶ S)
  (π' :
    T' ⟶
      tensorObj (𝒞 :=
        CompHausLike.category fun (X : TopCat.{0}) ↦
          TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))
        S' ℕ∪{∞})
  (g' : T' ⟶ T) (hπ' : Epi π') (hy' : Epi y') (comp : π' ≫ y' ▷ ℕ∪{∞} = g' ≫ π) (epi : Epi (cover π'))
  (split :
    SplitEpi (X := LightProfinite.fibre ∞ (π' ≫ snd S' ℕ∪{∞}))
      (LightProfinite.fibreIncl ∞ (π' ≫ snd S' ℕ∪{∞}) ≫ π' ≫ fst S' ℕ∪{∞})),
  ∃ (S' : LightProfinite.{0}) (π : S' ⟶ S) (_ :
    Function.Surjective (α :=
      (fun
            (X :
              CompHausLike fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) ↦
          (↑(X.toTop (P := fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
            Type))
        S')
      (β :=
      (fun
            (X :
              CompHausLike fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) ↦
          (↑(X.toTop (P := fun (X : TopCat.{0}) ↦
                TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
            Type))
        S)
      (⇑(ConcreteCategory.hom (FC :=
            fun
              (x1 x2 :
                CompHausLike fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) ↦
            C((↑(x1.toTop (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
                Type),
              (↑(x2.toTop (P := fun (X : TopCat.{0}) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
                Type)))
            (CC :=
            fun
              (X :
                CompHausLike fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) ↦
            (↑(X.toTop (P := fun (X : TopCat.{0}) ↦
                  TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))) :
              Type))
            π) :
        (a :
            (fun
                  (X :
                    CompHausLike fun (X : TopCat) ↦
                      TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) ↦
                (↑X.toTop : Type))
              S') →
          (fun
                (X :
                  CompHausLike fun (X : TopCat) ↦
                    TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type)) ↦
              (↑X.toTop : Type))
            S))
    (g' :
    obj (free R)
        (tensorObj (𝒞 :=
            CompHausLike.category fun (X : TopCat.{0}) ↦
              TotallyDisconnectedSpace (↑X : Type) ∧ SecondCountableTopology (↑X : Type))
            S' ℕ∪{∞}).toCondensed ⟶
      X),
    CategoryTheory.Functor.map (free R) (lightProfiniteToLightCondSet.map (π ▷ ℕ∪{∞})) ≫ f = g' ≫ p
Tactic: obtain ⟨S', T', y', π', g', hπ', hy', comp, ⟨⟨split⟩⟩, epi⟩ := aux π