-- Lean 4 proof step from Mathlib.Condensed.Light.Epi (goal)
Goal:
X Y : LightCondSet
f : X ⟶ Y
⊢ Epi f ↔
    ∀ (S : LightProfinite.{u})
      (y : Y.obj (P := Presheaf.IsSheaf (coherentTopology LightProfinite)).obj (Opposite.op S)),
      ∃ (S' : LightProfinite.{u}) (φ : S' ⟶ S) (_ :
        Function.Surjective (α :=
          (fun
                (X :
                  CompHausLike fun (X : TopCat.{u}) ↦
                    TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u)) ↦
              (↑(X.toTop (P := fun (X : TopCat.{u}) ↦
                    TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u))) :
                Type u))
            S')
          (β :=
          (fun
                (X :
                  CompHausLike fun (X : TopCat.{u}) ↦
                    TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u)) ↦
              (↑(X.toTop (P := fun (X : TopCat.{u}) ↦
                    TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u))) :
                Type u))
            S)
          (⇑(ConcreteCategory.hom (FC :=
                fun
                  (x1 x2 :
                    CompHausLike fun (X : TopCat.{u}) ↦
                      TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u)) ↦
                C((↑(x1.toTop (P := fun (X : TopCat.{u}) ↦
                        TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u))) :
                    Type u),
                  (↑(x2.toTop (P := fun (X : TopCat.{u}) ↦
                        TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u))) :
                    Type u)))
                (CC :=
                fun
                  (X :
                    CompHausLike fun (X : TopCat.{u}) ↦
                      TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u)) ↦
                (↑(X.toTop (P := fun (X : TopCat.{u}) ↦
                      TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u))) :
                  Type u))
                φ) :
            (a :
                (fun
                      (X :
                        CompHausLike fun (X : TopCat) ↦
                          TotallyDisconnectedSpace (↑X : Type u) ∧ SecondCountableTopology (↑X : Type u)) ↦
                    (↑X.toTop : Type u))
                  S') →
              (fun
                    (X :
                      CompHausLike fun (X : TopCat) ↦
                        TotallyDisconnectedSpace (↑X : Type u) ∧ SecondCountableTopology (↑X : Type u)) ↦
                  (↑X.toTop : Type u))
                S))
        (x : X.obj (P := Presheaf.IsSheaf (coherentTopology LightProfinite)).obj (Opposite.op S')),
        Eq (α :=
          (fun (X : Type u) ↦ X) (Y.obj (P := Presheaf.IsSheaf (coherentTopology LightProfinite)).obj (Opposite.op S')))
          ((ConcreteCategory.hom (FC := TypeCat.Fun) (f.hom.app (Opposite.op S')) :
                (a : (fun (X : Type u) ↦ X) (X.obj.obj (Opposite.op S'))) →
                  (fun (X : Type u) ↦ X) (Y.obj.obj (Opposite.op S')))
              x :
            (fun (X : Type u) ↦ X)
              (Y.obj (P := Presheaf.IsSheaf (coherentTopology LightProfinite)).obj (Opposite.op S')))
          ((ConcreteCategory.hom (FC := TypeCat.Fun)
                  (Y.obj (P := Presheaf.IsSheaf (coherentTopology LightProfinite)).map (Opposite.op φ)) :
                (a : (fun (X : Type u) ↦ X) (Y.obj.obj (Opposite.op S))) →
                  (fun (X : Type u) ↦ X) (Y.obj.obj (Opposite.op S')))
              y :
            (fun (X : Type u) ↦ X)
              (Y.obj (P := Presheaf.IsSheaf (coherentTopology LightProfinite)).obj (Opposite.op S')))
have rw : IsLocallySurjective (FA := TypeCat.Fun) f ↔
  ∀ (S : LightProfinite.{u}) (y : Y.obj (P := Presheaf.IsSheaf (coherentTopology LightProfinite)).obj (Opposite.op S)),
    ∃ (S' : LightProfinite.{u}) (φ : S' ⟶ S) (_ :
      Function.Surjective (α :=
        (fun
              (X :
                CompHausLike fun (X : TopCat.{u}) ↦
                  TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u)) ↦
            (↑(X.toTop (P := fun (X : TopCat.{u}) ↦
                  TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u))) :
              Type u))
          S')
        (β :=
        (fun
              (X :
                CompHausLike fun (X : TopCat.{u}) ↦
                  TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u)) ↦
            (↑(X.toTop (P := fun (X : TopCat.{u}) ↦
                  TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u))) :
              Type u))
          S)
        (⇑(ConcreteCategory.hom (FC :=
              fun
                (x1 x2 :
                  CompHausLike fun (X : TopCat.{u}) ↦
                    TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u)) ↦
              C((↑(x1.toTop (P := fun (X : TopCat.{u}) ↦
                      TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u))) :
                  Type u),
                (↑(x2.toTop (P := fun (X : TopCat.{u}) ↦
                      TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u))) :
                  Type u)))
              (CC :=
              fun
                (X :
                  CompHausLike fun (X : TopCat.{u}) ↦
                    TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u)) ↦
              (↑(X.toTop (P := fun (X : TopCat.{u}) ↦
                    TotallyDisconnectedSpace.{u} (↑X : Type u) ∧ SecondCountableTopology.{u} (↑X : Type u))) :
                Type u))
              φ) :
          (a :
              (fun
                    (X :
                      CompHausLike fun (X : TopCat) ↦
                        TotallyDisconnectedSpace (↑X : Type u) ∧ SecondCountableTopology (↑X : Type u)) ↦
                  (↑X.toTop : Type u))
                S') →
            (fun
                  (X :
                    CompHausLike fun (X : TopCat) ↦
                      TotallyDisconnectedSpace (↑X : Type u) ∧ SecondCountableTopology (↑X : Type u)) ↦
                (↑X.toTop : Type u))
              S))
      (x : X.obj (P := Presheaf.IsSheaf (coherentTopology LightProfinite)).obj (Opposite.op S')),
      Eq (α :=
        (fun (X : Type u) ↦ X) (Y.obj (P := Presheaf.IsSheaf (coherentTopology LightProfinite)).obj (Opposite.op S')))
        ((ConcreteCategory.hom (FC := TypeCat.Fun) (f.hom.app (Opposite.op S')) :
              (a : (fun (X : Type u) ↦ X) (X.obj.obj (Opposite.op S'))) →
                (fun (X : Type u) ↦ X) (Y.obj.obj (Opposite.op S')))
            x :
          (fun (X : Type u) ↦ X) (Y.obj (P := Presheaf.IsSheaf (coherentTopology LightProfinite)).obj (Opposite.op S')))
        ((ConcreteCategory.hom (FC := TypeCat.Fun)
                (Y.obj (P := Presheaf.IsSheaf (coherentTopology LightProfinite)).map (Opposite.op φ)) :
              (a : (fun (X : Type u) ↦ X) (Y.obj.obj (Opposite.op S))) →
                (fun (X : Type u) ↦ X) (Y.obj.obj (Opposite.op S')))
            y :
          (fun (X : Type u) ↦ X) (Y.obj (P := Presheaf.IsSheaf (coherentTopology LightProfinite)).obj (Opposite.op S')))
Tactic: rw [← isLocallySurjective_iff_epi']