-- Lean 4 proof step from Mathlib.Condensed.Discrete.LocallyConstant (goal)
Goal:
P : TopCat.{u} → Prop
inst : ∀ (S : CompHausLike P) (p : (↑(S.toTop (P := P)) : Type u) → Prop), HasProp P (Subtype p)
S : CompHausLike P
Y : (CompHausLike P)ᵒᵖ ⥤ Type (max u w)
inst_1 : HasProp P PUnit.{u + 1}
f : LocallyConstant (↑(S.toTop (P := P)) : Type u) (Y.obj (op (of P PUnit.{u + 1})))
T : CompHausLike P
g : T ⟶ S
X_1 : TopCat
inst_2 : HasExplicitFiniteCoproducts P
inst_3 : HasExplicitPullbacks P
hs :
  ∀ ⦃X Y : CompHausLike P⦄ (f : X ⟶ Y),
    EffectiveEpi f →
      Surjective (α := (fun (X : CompHausLike P) ↦ (↑(X.toTop (P := P)) : Type u)) X) (β :=
        (fun (X : CompHausLike P) ↦ (↑(X.toTop (P := P)) : Type u)) Y)
        (⇑(ConcreteCategory.hom (FC := fun (x1 x2 : CompHausLike P) ↦
              C((↑(x1.toTop (P := P)) : Type u), (↑(x2.toTop (P := P)) : Type u))) (CC := fun (X : CompHausLike P) ↦
              (↑(X.toTop (P := P)) : Type u)) f) :
          (a : (fun (X : CompHausLike P) ↦ (↑X.toTop : Type u)) X) → (fun (X : CompHausLike P) ↦ (↑X.toTop : Type u)) Y)
inst_4 : HasExplicitFiniteCoproducts P
X : Sheaf (C := CompHausLike P) (coherentTopology (CompHausLike P)) (Type (max u w))
x : X.obj (P := Presheaf.IsSheaf (coherentTopology (CompHausLike P))).obj (op (of P PUnit.{u + 1}))
⊢ TypeCat.Fun.toFun
      (ConcreteCategory.hom (FC := TypeCat.Fun)
        (CategoryStruct.comp (Y :=
          Functor.obj (C := Sheaf (C := CompHausLike P) (coherentTopology (CompHausLike P)) (Type (max u w)))
            (Functor.obj (sheafSections (coherentTopology (CompHausLike P)) (Type (max u w)))
                (op (of P PUnit.{u + 1})) :
              Sheaf (C := CompHausLike P) (coherentTopology (CompHausLike P)) (Type (max u w)) ⥤ Type (max u w))
            (Functor.obj
                (functor.{u, w} P hs :
                  Type (max u w) ⥤ Sheaf (C := CompHausLike P) (coherentTopology (CompHausLike P)) (Type (max u w)))
                (Functor.obj
                  (Functor.obj (sheafSections (coherentTopology (CompHausLike P)) (Type (max u w)))
                    (op (of P PUnit.{u + 1})))
                  X) :
              Sheaf (C := CompHausLike P) (coherentTopology (CompHausLike P)) (Type (max u w))))
          (NatTrans.app (unit P hs)
              (Functor.obj
                (Functor.obj (sheafSections (coherentTopology (CompHausLike P)) (Type (max u w)))
                  (op (of P PUnit.{u + 1})))
                X) :
            Functor.obj (𝟭 (Type (max u w)))
                (Functor.obj
                  (Functor.obj (sheafSections (coherentTopology (CompHausLike P)) (Type (max u w)))
                    (op (of P PUnit.{u + 1})))
                  X) ⟶
              (Functor.comp (D := Sheaf (C := CompHausLike P) (coherentTopology (CompHausLike P)) (Type (max u w)))
                    (functor.{u, w} P hs :
                      Type (max u w) ⥤ Sheaf (C := CompHausLike P) (coherentTopology (CompHausLike P)) (Type (max u w)))
                    (Functor.obj (sheafSections (coherentTopology (CompHausLike P)) (Type (max u w)))
                        (op (of P PUnit.{u + 1})) :
                      Sheaf (C := CompHausLike P) (coherentTopology (CompHausLike P)) (Type (max u w)) ⥤
                        Type (max u w))).obj
                (Functor.obj
                  (Functor.obj (sheafSections (coherentTopology (CompHausLike P)) (Type (max u w)))
                    (op (of P PUnit.{u + 1})))
                  X))
          (CategoryTheory.Functor.map
            (Functor.obj (sheafSections (coherentTopology (CompHausLike P)) (Type (max u w))) (op (of P PUnit.{u + 1})))
            (NatTrans.app (counit P hs) X))))
      x =
    (ConcreteCategory.hom (FC := TypeCat.Fun)
          (𝟙
            (Functor.obj
              (Functor.obj (sheafSections (coherentTopology (CompHausLike P)) (Type (max u w)))
                (op (of P PUnit.{u + 1})))
              X))).toFun
      x
have h : ((@ConcreteCategory.hom _ types TypeCat.Fun _ _ _ _ _
          (NatTrans.app (InducedCategory.Hom.hom (NatTrans.app (counit P hs) X)) (op (of P PUnit.{u + 1}))) :
        TypeCat.Fun
          (LocallyConstant PUnit.{u + 1}
            (X.obj (P := Presheaf.IsSheaf (coherentTopology (CompHausLike P))).obj (op (of P PUnit.{u + 1}))))
          (X.obj (P := Presheaf.IsSheaf (coherentTopology (CompHausLike P))).obj (op (of P PUnit.{u + 1})))) :
      (a : LocallyConstant PUnit.{u + 1} (X.obj.obj (op (of P PUnit.{u + 1})))) → X.obj.obj (op (of P PUnit.{u + 1})))
    (LocallyConstant.const PUnit.{u + 1} x) =
  x
Tactic: dsimp