-- Lean 4 proof step from Mathlib.AlgebraicGeometry.StructureSheaf (goal)
Goal:
R M A : Type u
inst : CommRing R
inst_1 : AddCommGroup M
inst_2 : Module R M
inst_3 : CommRing A
inst_4 : Algebra R A
x : (↑(PrimeSpectrum.Top R) : Type u)
m : (↑(ModuleCat.of R M) : Type u)
⊢ Eq (α := (↑(stalk (structurePresheafInModuleCat R M) x) : Type u))
    (((ModuleCat.Hom.hom (germ (structurePresheafInModuleCat R M) ⊤ x _root_.trivial) :
            (↑(ModuleCat.of R
                    (Functor.obj (C := (Opens (↑(PrimeSpectrum.Top R) : Type u))ᵒᵖ)
                      (ObjectProperty.FullSubcategory.obj (P :=
                        CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑(PrimeSpectrum.Top R) : Type u)))
                        (structureSheafInType R M))
                      (op ⊤))) :
                Type u) →ₗ[R]
              (↑(stalk (structurePresheafInModuleCat R M) x) : Type u)) :
          (a : (structureSheafInType R M).obj.obj (op ⊤)) → (↑((structurePresheafInModuleCat R M).stalk x) : Type u))
        (const m 1 ⊤ ⋯) :
      (↑(stalk (structurePresheafInModuleCat R M) x) : Type u))
    (((ConcreteCategory.hom (FC := fun (x1 x2 : ModuleCat R) ↦ (↑x1 : Type u) →ₗ[R] (↑x2 : Type u)) (CC :=
              ModuleCat.carrier) (X := Functor.obj (structurePresheafInModuleCat R M) (op (basicOpen 1))) (Y :=
              stalk (structurePresheafInModuleCat R M) x) (germ (structurePresheafInModuleCat R M) (basicOpen 1) x ⋯) :
            (↑(Functor.obj (structurePresheafInModuleCat R M) (op (basicOpen 1))) : Type u) →ₗ[R]
              (↑(stalk (structurePresheafInModuleCat R M) x) : Type u)) :
          (a : (structureSheafInType R M).obj.obj (op (basicOpen 1))) →
            (↑((structurePresheafInModuleCat R M).stalk x) : Type u))
        (const m 1 (basicOpen 1) ⋯) :
      (↑(stalk (structurePresheafInModuleCat R M) x) : Type u))
have h : Eq (α := (↑(stalk (structurePresheafInModuleCat R M) x) : Type u))
  (((ModuleCat.Hom.hom (germ (structurePresheafInModuleCat R M) ⊤ x _root_.trivial) :
          (↑(ModuleCat.of R
                  (Functor.obj (C := (Opens (↑(PrimeSpectrum.Top R) : Type u))ᵒᵖ)
                    (ObjectProperty.FullSubcategory.obj (P :=
                      CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑(PrimeSpectrum.Top R) : Type u)))
                      (structureSheafInType R M))
                    (op ⊤))) :
              Type u) →ₗ[R]
            (↑(stalk (structurePresheafInModuleCat R M) x) : Type u)) :
        (a : (structureSheafInType R M).obj.obj (op ⊤)) → (↑((structurePresheafInModuleCat R M).stalk x) : Type u))
      (const m 1 ⊤ sorry) :
    (↑(stalk (structurePresheafInModuleCat R M) x) : Type u))
  (((ConcreteCategory.hom (FC := fun (x1 x2 : ModuleCat R) ↦ (↑x1 : Type u) →ₗ[R] (↑x2 : Type u)) (CC :=
            ModuleCat.carrier) (X := Functor.obj (structurePresheafInModuleCat R M) (op ⊤)) (Y :=
            stalk (structurePresheafInModuleCat R M) x) (germ (structurePresheafInModuleCat R M) ⊤ x sorry) :
          (↑(Functor.obj (structurePresheafInModuleCat R M) (op ⊤)) : Type u) →ₗ[R]
            (↑(stalk (structurePresheafInModuleCat R M) x) : Type u)) :
        (a : (structureSheafInType R M).obj.obj (op ⊤)) → (↑((structurePresheafInModuleCat R M).stalk x) : Type u))
      (const m 1 ⊤ sorry) :
    (↑(stalk (structurePresheafInModuleCat R M) x) : Type u))
Tactic: rw! [PrimeSpectrum.basicOpen_one]