-- 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)
r : R
m : M
⊢ Eq (α := (↑(stalk (PresheafOfModules.presheaf (moduleStructurePresheaf R M)) x) : Type u))
    ((ConcreteCategory.hom (FC := fun (x1 x2 : AddCommGrpCat.{u}) ↦ (↑x1 : Type u) →+ (↑x2 : Type u)) (CC :=
            AddCommGrpCat.carrier) (X :=
            Functor.obj (C := (Opens (↑(PrimeSpectrum.Top R) : Type u))ᵒᵖ)
              (PresheafOfModules.presheaf (moduleStructurePresheaf R M)) (op ⊤))
            (Y := stalk (PresheafOfModules.presheaf (moduleStructurePresheaf R M)) x)
            (germ (PresheafOfModules.presheaf (moduleStructurePresheaf R M)) ⊤ x ⋯) :
          (a : (↑((moduleStructurePresheaf R M).presheaf.obj (op ⊤)) : Type u)) →
            (↑(stalk (moduleStructurePresheaf R M).presheaf x) : Type u))
        (HSMul.hSMul (β :=
          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 ⊤))
          r
          ((toOpenₗ R M ⊤ : (a : M) → (structureSheafInType R M).obj.obj (op ⊤)) m :
            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 ⊤))) :
      (↑(stalk (PresheafOfModules.presheaf (moduleStructurePresheaf R M)) x) : Type u))
    ((ConcreteCategory.hom (FC := fun (x1 x2 : AddCommGrpCat.{u}) ↦ (↑x1 : Type u) →+ (↑x2 : Type u)) (CC :=
            AddCommGrpCat.carrier) (X :=
            Functor.obj (C := (Opens (↑(PrimeSpectrum.Top R) : Type u))ᵒᵖ)
              (PresheafOfModules.presheaf (moduleStructurePresheaf R M)) (op ⊤))
            (Y := stalk (PresheafOfModules.presheaf (moduleStructurePresheaf R M)) x)
            (germ (PresheafOfModules.presheaf (moduleStructurePresheaf R M)) ⊤ x _root_.trivial) :
          (a : (↑((moduleStructurePresheaf R M).presheaf.obj (op ⊤)) : Type u)) →
            (↑(stalk (moduleStructurePresheaf R M).presheaf x) : Type u))
        (HSMul.hSMul (α :=
          (↑(Functor.obj (C := (Opens (↑(PrimeSpectrum.Top R) : Type u))ᵒᵖ) (structurePresheafInCommRingCat R) (op ⊤)) :
            Type u))
          (β := (↑(PresheafOfModules.obj (moduleStructurePresheaf R M) (op ⊤)) : Type u))
          ((CommRingCat.Hom.hom'
                  (CommRingCat.ofHom
                    (algebraMap 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 R))
                        (op ⊤)))) :
                (a : (↑(CommRingCat.of R) : Type u)) →
                  (↑(CommRingCat.of ((structureSheafInType R R).obj.obj (op ⊤))) : Type u))
              r :
            (↑(CommRingCat.of
                  (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 R))
                    (op ⊤))) :
              Type u))
          ((toOpenₗ R M ⊤ : (a : M) → (structureSheafInType R M).obj.obj (op ⊤)) m :
            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 ⊤))) :
      (↑(stalk (PresheafOfModules.presheaf (moduleStructurePresheaf R M)) x) : Type u))
have h : HSMul.hSMul (β :=
    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 ⊤))
    r
    ((toOpenₗ R M ⊤ : (a : M) → (structureSheafInType R M).obj.obj (op ⊤)) m :
      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 ⊤)) =
  HSMul.hSMul (α :=
    (↑(Functor.obj (C := (Opens (↑(PrimeSpectrum.Top R) : Type u))ᵒᵖ) (structurePresheafInCommRingCat R) (op ⊤)) :
      Type u))
    (β := (↑(PresheafOfModules.obj (moduleStructurePresheaf R M) (op ⊤)) : Type u))
    ((CommRingCat.Hom.hom'
            (CommRingCat.ofHom
              (algebraMap 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 R))
                  (op ⊤)))) :
          (a : (↑(CommRingCat.of R) : Type u)) →
            (↑(CommRingCat.of ((structureSheafInType R R).obj.obj (op ⊤))) : Type u))
        r :
      (↑(CommRingCat.of
            (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 R))
              (op ⊤))) :
        Type u))
    ((toOpenₗ R M ⊤ : (a : M) → (structureSheafInType R M).obj.obj (op ⊤)) m :
      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 ⊤))
Tactic: congr 1