-- Lean 4 proof step from Mathlib.AlgebraicGeometry.ProjectiveSpectrum.Scheme (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
f : A
x : ↥(pbo f)
m : ℕ
f_deg : f ∈ 𝒜 m
hm : 0 < m
this : Algebra (A⁰_ f) (AtPrime 𝒜 (↑x : ProjectiveSpectrum 𝒜).asHomogeneousIdeal.toIdeal) :=
  RingHom.toAlgebra (mapId 𝒜 ⋯)
y z : NumDenSameDeg 𝒜 (Submonoid.powers f)
e :
  Eq (α := AtPrime 𝒜 (↑x : ProjectiveSpectrum 𝒜).asHomogeneousIdeal.toIdeal)
    ((algebraMap (A⁰_ f) (AtPrime 𝒜 (↑x : ProjectiveSpectrum 𝒜).asHomogeneousIdeal.toIdeal) :
          (a : A⁰_ f) → AtPrime 𝒜 (↑x : ProjectiveSpectrum 𝒜).asHomogeneousIdeal.toIdeal)
        (HomogeneousLocalization.mk y) :
      AtPrime 𝒜 (↑x : ProjectiveSpectrum 𝒜).asHomogeneousIdeal.toIdeal)
    ((algebraMap (A⁰_ f) (AtPrime 𝒜 (↑x : ProjectiveSpectrum 𝒜).asHomogeneousIdeal.toIdeal) :
          (a : A⁰_ f) → AtPrime 𝒜 (↑x : ProjectiveSpectrum 𝒜).asHomogeneousIdeal.toIdeal)
        (HomogeneousLocalization.mk z) :
      AtPrime 𝒜 (↑x : ProjectiveSpectrum 𝒜).asHomogeneousIdeal.toIdeal)
⊢ ∃ (c :
    ↥(Ideal.primeCompl
        (PrimeSpectrum.asIdeal
          ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{u_1}) ↦ C((↑X : Type u_1), (↑Y : Type u_1))) (CC := carrier)
                (X :=
                (↑(Proj.restrict (U := Functor.obj (C := Opens (↑Proj.T : Type u_1)) (Opens.toTopCat Proj.T) (pbo f))
                        ⋯).toPresheafedSpace :
                  TopCat))
                (PresheafedSpace.Hom.base (LocallyRingedSpace.Hom.toHom (toSpec 𝒜 f))) :
              (a : (↑(↑(Proj.restrict ⋯).toPresheafedSpace : TopCat) : Type u_1)) →
                (↑(↑(Spec A⁰_ f).toPresheafedSpace : TopCat) : Type u_1))
            x)))),
    (↑c : A⁰_ f) * HomogeneousLocalization.mk y = (↑c : A⁰_ f) * HomogeneousLocalization.mk z
have toIsLocalizationMap : ∀ (i : ℕ) (c : A) (hc : c ∈ 𝒜 i) (hc' : c ∉ (↑x : ProjectiveSpectrum 𝒜).asHomogeneousIdeal)
  (e : c * ((↑z.den : A) * (↑y.num : A)) = c * ((↑y.den : A) * (↑z.num : A))),
  ∃ (c :
    ↥(Ideal.primeCompl
        (PrimeSpectrum.asIdeal
          ((ConcreteCategory.hom (FC := fun (X Y : TopCat.{u_1}) ↦ C((↑X : Type u_1), (↑Y : Type u_1))) (CC := carrier)
                (X :=
                (↑(Proj.restrict (U := Functor.obj (C := Opens (↑Proj.T : Type u_1)) (Opens.toTopCat Proj.T) (pbo f))
                        sorry).toPresheafedSpace :
                  TopCat))
                (PresheafedSpace.Hom.base (LocallyRingedSpace.Hom.toHom (toSpec 𝒜 f))) :
              (a : (↑(↑(Proj.restrict sorry).toPresheafedSpace : TopCat) : Type u_1)) →
                (↑(↑(Spec A⁰_ f).toPresheafedSpace : TopCat) : Type u_1))
            x)))),
    (↑c : A⁰_ f) * HomogeneousLocalization.mk y = (↑c : A⁰_ f) * HomogeneousLocalization.mk z
Tactic: obtain ⟨i, c, hc, hc', e⟩ : ∃ i, ∃ c ∈ 𝒜 i, c ∉ x.1.asHomogeneousIdeal ∧
        c * (z.den.1 * y.num.1) = c * (y.den.1 * z.num.1) := by
      apply_fun HomogeneousLocalization.val at e
      simp only [RingHom.algebraMap_toAlgebra, map_mk, GradedRingHom.id_apply, val_mk, mk_eq_mk',
        IsLocalization.mk'_eq_iff_eq] at e
      obtain ⟨⟨c, hcx⟩, hc⟩ := IsLocalization.exists_of_eq (M := x.1.1.toIdeal.primeCompl) e
      obtain ⟨i, hi⟩ := not_forall.mp ((x.1.1.isHomogeneous.mem_iff _).not.mp hcx)
      refine ⟨i, _, (decompose 𝒜 c i).2, hi, ?_⟩
      apply_fun fun x ↦ (decompose 𝒜 x (i + z.deg + y.deg)).1 at hc
      conv_rhs at hc => rw [add_right_comm]
      rwa [← mul_assoc, coe_decompose_mul_add_of_right_mem, coe_decompose_mul_add_of_right_mem,
        ← mul_assoc, coe_decompose_mul_add_of_right_mem, coe_decompose_mul_add_of_right_mem,
        mul_assoc, mul_assoc] at hc
      exacts [y.den.2, z.num.2, z.den.2, y.num.2]