-- Lean 4 proof step from Mathlib.AlgebraicGeometry.ProjectiveSpectrum.StructureSheaf (goal)
Goal:
A : Type u_1
σ : Type u_2
inst : CommRing A
inst_1 : SetLike σ A
inst_2 : AddSubgroupClass σ A
𝒜 : ℕ → σ
inst_3 : GradedRing 𝒜
x : (↑(ProjectiveSpectrum.top 𝒜) : Type u_1)
z : HomogeneousLocalization.NumDenSameDeg 𝒜 x.asHomogeneousIdeal.toIdeal.primeCompl
rw :
  Eq (α := (↑(CommRingCat.of (at x)) : Type u_1))
    ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u_1}) ↦ (↑R : Type u_1) →+* (↑S : Type u_1)) (CC :=
          CommRingCat.carrier) (X := Presheaf.stalk (Sheaf.presheaf (Proj.structureSheaf 𝒜)) x)
          (stalkToFiberRingHom 𝒜 x) :
        (a : (↑((Proj.structureSheaf 𝒜).presheaf.stalk x) : Type u_1)) → (↑(CommRingCat.of (at x)) : Type u_1))
      (Quotient.liftOn' (s₁ :=
        Setoid.ker (HomogeneousLocalization.NumDenSameDeg.embedding 𝒜 x.asHomogeneousIdeal.toIdeal.primeCompl))
        (Quotient.mk'' z)
        (fun (f : HomogeneousLocalization.NumDenSameDeg 𝒜 x.asHomogeneousIdeal.toIdeal.primeCompl) ↦
          ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u_1}) ↦ (↑R : Type u_1) →+* (↑S : Type u_1)) (CC :=
                  CommRingCat.carrier) (X :=
                  Functor.obj (C := (Opens (↑(ProjectiveSpectrum.top 𝒜) : Type u_1))ᵒᵖ)
                    (Sheaf.presheaf (Proj.structureSheaf 𝒜) : Presheaf CommRingCat (ProjectiveSpectrum.top 𝒜))
                    (op (ProjectiveSpectrum.basicOpen 𝒜 (↑f.den : A)) :
                      (Opens (↑(ProjectiveSpectrum.top 𝒜) : Type u_1))ᵒᵖ))
                  (Y := Presheaf.stalk (Sheaf.presheaf (Proj.structureSheaf 𝒜)) x)
                  (Presheaf.germ (Sheaf.presheaf (Proj.structureSheaf 𝒜)) (ProjectiveSpectrum.basicOpen 𝒜 (↑f.den : A))
                      x ⋯ :
                    Functor.obj (C := (Opens (↑(ProjectiveSpectrum.top 𝒜) : Type u_1))ᵒᵖ)
                        (Sheaf.presheaf (Proj.structureSheaf 𝒜) : Presheaf CommRingCat (ProjectiveSpectrum.top 𝒜))
                        (op (ProjectiveSpectrum.basicOpen 𝒜 (↑f.den : A)) :
                          (Opens (↑(ProjectiveSpectrum.top 𝒜) : Type u_1))ᵒᵖ) ⟶
                      Presheaf.stalk (Sheaf.presheaf (Proj.structureSheaf 𝒜)) x) :
                (↑(Functor.obj (C := (Opens (↑(ProjectiveSpectrum.top 𝒜) : Type u_1))ᵒᵖ)
                        (Sheaf.presheaf (Proj.structureSheaf 𝒜) : Presheaf CommRingCat (ProjectiveSpectrum.top 𝒜))
                        (op (ProjectiveSpectrum.basicOpen 𝒜 (↑f.den : A)) :
                          (Opens (↑(ProjectiveSpectrum.top 𝒜) : Type u_1))ᵒᵖ)) :
                    Type u_1) →+*
                  (↑(Presheaf.stalk (Sheaf.presheaf (Proj.structureSheaf 𝒜)) x) : Type u_1)) :
              (a :
                  (↑((Proj.structureSheaf 𝒜).presheaf.obj (op (ProjectiveSpectrum.basicOpen 𝒜 (↑f.den : A)))) :
                    Type u_1)) →
                (↑((Proj.structureSheaf 𝒜).presheaf.stalk x) : Type u_1))
            (sectionInBasicOpen 𝒜 x f))
        ⋯))
    (Quotient.mk'' z)
⊢ Eq (α := (↑(CommRingCat.of (at x)) : Type u_1))
    ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u_1}) ↦ (↑R : Type u_1) →+* (↑S : Type u_1)) (CC :=
            CommRingCat.carrier) (X := Presheaf.stalk (Sheaf.presheaf (Proj.structureSheaf 𝒜)) x)
            (stalkToFiberRingHom 𝒜 x) :
          (a : (↑((Proj.structureSheaf 𝒜).presheaf.stalk x) : Type u_1)) → (↑(CommRingCat.of (at x)) : Type u_1))
        (homogeneousLocalizationToStalk 𝒜 x (Quotient.mk'' z)) :
      (↑(CommRingCat.of (at x)) : Type u_1))
    (Quotient.mk'' z :
      _root_.Quotient (α := HomogeneousLocalization.NumDenSameDeg 𝒜 x.asHomogeneousIdeal.toIdeal.primeCompl)
        (Setoid.ker (HomogeneousLocalization.NumDenSameDeg.embedding 𝒜 x.asHomogeneousIdeal.toIdeal.primeCompl)))
have rw₁ : Eq (α := (↑(CommRingCat.of (at x)) : Type u_1))
  ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u_1}) ↦ (↑R : Type u_1) →+* (↑S : Type u_1)) (CC :=
          CommRingCat.carrier) (X := Presheaf.stalk (Sheaf.presheaf (Proj.structureSheaf 𝒜)) x)
          (stalkToFiberRingHom 𝒜 x) :
        (a : (↑((Proj.structureSheaf 𝒜).presheaf.stalk x) : Type u_1)) → (↑(CommRingCat.of (at x)) : Type u_1))
      (((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u_1}) ↦ (↑R : Type u_1) →+* (↑S : Type u_1)) (CC :=
              CommRingCat.carrier) (X :=
              Functor.obj (C := (Opens (↑(ProjectiveSpectrum.top 𝒜) : Type u_1))ᵒᵖ)
                (Sheaf.presheaf (Proj.structureSheaf 𝒜) : Presheaf CommRingCat (ProjectiveSpectrum.top 𝒜))
                (op (ProjectiveSpectrum.basicOpen 𝒜 (↑z.den : A)) : (Opens (↑(ProjectiveSpectrum.top 𝒜) : Type u_1))ᵒᵖ))
              (Y := Presheaf.stalk (Sheaf.presheaf (Proj.structureSheaf 𝒜)) x)
              (Presheaf.germ (Sheaf.presheaf (Proj.structureSheaf 𝒜)) (ProjectiveSpectrum.basicOpen 𝒜 (↑z.den : A)) x
                  sorry :
                Functor.obj (C := (Opens (↑(ProjectiveSpectrum.top 𝒜) : Type u_1))ᵒᵖ)
                    (Sheaf.presheaf (Proj.structureSheaf 𝒜) : Presheaf CommRingCat (ProjectiveSpectrum.top 𝒜))
                    (op (ProjectiveSpectrum.basicOpen 𝒜 (↑z.den : A)) :
                      (Opens (↑(ProjectiveSpectrum.top 𝒜) : Type u_1))ᵒᵖ) ⟶
                  Presheaf.stalk (Sheaf.presheaf (Proj.structureSheaf 𝒜)) x) :
            (↑(Functor.obj (C := (Opens (↑(ProjectiveSpectrum.top 𝒜) : Type u_1))ᵒᵖ)
                    (Sheaf.presheaf (Proj.structureSheaf 𝒜) : Presheaf CommRingCat (ProjectiveSpectrum.top 𝒜))
                    (op (ProjectiveSpectrum.basicOpen 𝒜 (↑z.den : A)) :
                      (Opens (↑(ProjectiveSpectrum.top 𝒜) : Type u_1))ᵒᵖ)) :
                Type u_1) →+*
              (↑(Presheaf.stalk (Sheaf.presheaf (Proj.structureSheaf 𝒜)) x) : Type u_1)) :
          (a :
              (↑((Proj.structureSheaf 𝒜).presheaf.obj (op (ProjectiveSpectrum.basicOpen 𝒜 (↑z.den : A)))) : Type u_1)) →
            (↑((Proj.structureSheaf 𝒜).presheaf.stalk x) : Type u_1))
        (sectionInBasicOpen 𝒜 x z)) :
    (↑(CommRingCat.of (at x)) : Type u_1))
  (Quotient.mk'' z :
    _root_.Quotient (α := HomogeneousLocalization.NumDenSameDeg 𝒜 x.asHomogeneousIdeal.toIdeal.primeCompl)
      (Setoid.ker (HomogeneousLocalization.NumDenSameDeg.embedding 𝒜 x.asHomogeneousIdeal.toIdeal.primeCompl)))
Tactic: rw [homogeneousLocalizationToStalk, Quotient.liftOn'_mk'',
    stalkToFiberRingHom_germ, sectionInBasicOpen]