-- Lean 4 proof step from Mathlib.AlgebraicGeometry.Modules.Tilde (goal)
Goal:
R : CommRingCat
M_1 : ModuleCat (↑R : Type u)
M N : (Spec R).Modules
f : Functor.obj modulesSpecToSheaf M ⟶ Functor.obj modulesSpecToSheaf N
U : (Opens ↥(Spec R))ᵒᵖ
t :
  (↑(Functor.obj
        (ObjectProperty.FullSubcategory.obj (P :=
          CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))) (Spec R).ringCatSheaf)
        U) :
    Type u)
m : (↑(M.val.1 U) : Type u)
x : ↥(Spec R)
hxU : x ∈ unop U
⊢ ∃ (V : Opens ↥(Spec R)) (hV : V ≤ unop U),
    x ∈ V ∧
      Eq (α :=
        (↑((Functor.obj modulesSpecToSheaf N).obj (P :=
                  CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))).obj
              (op V)) :
          Type u))
        ((ConcreteCategory.hom (FC := fun (x1 x2 : ModuleCat (↑R : Type u)) ↦
                LinearMap (R := (↑R : Type u)) (RingHom.id (↑R : Type u)) (↑x1 : Type u) (↑x2 : Type u)) (CC :=
                ModuleCat.carrier)
                ((Functor.obj modulesSpecToSheaf N).obj (P :=
                      CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))).map
                  (homOfLE hV).op) :
              (a : (↑((modulesSpecToSheaf.obj N).obj.obj (op (unop U))) : Type u)) →
                (↑((modulesSpecToSheaf.obj N).obj.obj (op V)) : Type u))
            ((ModuleCat.Hom.hom (f.hom.app U)).toFun (t • m)) :
          (↑((Functor.obj modulesSpecToSheaf N).obj (P :=
                    CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))).obj
                (op V)) :
            Type u))
        ((ConcreteCategory.hom (FC := fun (x1 x2 : ModuleCat (↑R : Type u)) ↦
                LinearMap (R := (↑R : Type u)) (RingHom.id (↑R : Type u)) (↑x1 : Type u) (↑x2 : Type u)) (CC :=
                ModuleCat.carrier)
                ((Functor.obj modulesSpecToSheaf N).obj (P :=
                      CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))).map
                  (homOfLE hV).op) :
              (a : (↑((modulesSpecToSheaf.obj N).obj.obj (op (unop U))) : Type u)) →
                (↑((modulesSpecToSheaf.obj N).obj.obj (op V)) : Type u))
            (HSMul.hSMul (α :=
              (↑(Functor.obj
                    (ObjectProperty.FullSubcategory.obj (P :=
                      CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))) (Spec R).ringCatSheaf)
                    U) :
                Type u))
              ((RingHom.id
                      (↑(Functor.obj
                            (ObjectProperty.FullSubcategory.obj (P :=
                              CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R)))
                              (Spec R).ringCatSheaf)
                            U) :
                        Type u) :
                    (a : (↑((Spec R).ringCatSheaf.obj.obj U) : Type u)) → (↑((Spec R).ringCatSheaf.obj.obj U) : Type u))
                  t :
                (↑(Functor.obj
                      (ObjectProperty.FullSubcategory.obj (P :=
                        CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))) (Spec R).ringCatSheaf)
                      U) :
                  Type u))
              ((ModuleCat.Hom.hom (f.hom.app U)).toFun m)) :
          (↑((Functor.obj modulesSpecToSheaf N).obj (P :=
                    CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))).obj
                (op V)) :
            Type u))
have obtain : ∀ (r : (↑R : Type u)) (hxr : x ∈ (↑(basicOpen r) : Set (PrimeSpectrum (↑R : Type u)))) (hrU : basicOpen r ≤ unop U),
  ∃ (V : Opens ↥(Spec R)) (hV : V ≤ unop U),
    x ∈ V ∧
      Eq (α :=
        (↑((Functor.obj modulesSpecToSheaf N).obj (P :=
                  CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))).obj
              (op V)) :
          Type u))
        ((ConcreteCategory.hom (FC := fun (x1 x2 : ModuleCat (↑R : Type u)) ↦
                LinearMap (R := (↑R : Type u)) (RingHom.id (↑R : Type u)) (↑x1 : Type u) (↑x2 : Type u)) (CC :=
                ModuleCat.carrier)
                ((Functor.obj modulesSpecToSheaf N).obj (P :=
                      CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))).map
                  (homOfLE hV).op) :
              (a : (↑((modulesSpecToSheaf.obj N).obj.obj (op (unop U))) : Type u)) →
                (↑((modulesSpecToSheaf.obj N).obj.obj (op V)) : Type u))
            ((ModuleCat.Hom.hom (f.hom.app U)).toFun (t • m)) :
          (↑((Functor.obj modulesSpecToSheaf N).obj (P :=
                    CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))).obj
                (op V)) :
            Type u))
        ((ConcreteCategory.hom (FC := fun (x1 x2 : ModuleCat (↑R : Type u)) ↦
                LinearMap (R := (↑R : Type u)) (RingHom.id (↑R : Type u)) (↑x1 : Type u) (↑x2 : Type u)) (CC :=
                ModuleCat.carrier)
                ((Functor.obj modulesSpecToSheaf N).obj (P :=
                      CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))).map
                  (homOfLE hV).op) :
              (a : (↑((modulesSpecToSheaf.obj N).obj.obj (op (unop U))) : Type u)) →
                (↑((modulesSpecToSheaf.obj N).obj.obj (op V)) : Type u))
            (HSMul.hSMul (α :=
              (↑(Functor.obj
                    (ObjectProperty.FullSubcategory.obj (P :=
                      CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))) (Spec R).ringCatSheaf)
                    U) :
                Type u))
              ((RingHom.id
                      (↑(Functor.obj
                            (ObjectProperty.FullSubcategory.obj (P :=
                              CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R)))
                              (Spec R).ringCatSheaf)
                            U) :
                        Type u) :
                    (a : (↑((Spec R).ringCatSheaf.obj.obj U) : Type u)) → (↑((Spec R).ringCatSheaf.obj.obj U) : Type u))
                  t :
                (↑(Functor.obj
                      (ObjectProperty.FullSubcategory.obj (P :=
                        CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))) (Spec R).ringCatSheaf)
                      U) :
                  Type u))
              ((ModuleCat.Hom.hom (f.hom.app U)).toFun m)) :
          (↑((Functor.obj modulesSpecToSheaf N).obj (P :=
                    CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology ↥(Spec R))).obj
                (op V)) :
            Type u))
Tactic: obtain ⟨a, ⟨_, ⟨r, rfl⟩, rfl⟩, hxr, hrU : basicOpen _ ≤ _⟩ :=
      PrimeSpectrum.isBasis_basic_opens.exists_subset_of_mem_open hxU U.unop.2