-- Lean 4 proof step from Mathlib.RingTheory.LocalProperties.Projective (goal)
Goal:
R : Type u_1
N : Type u_2
M : Type uM
inst : CommRing R
inst_1 : AddCommGroup M
inst_2 : Module R M
inst_3 : AddCommGroup N
inst_4 : Module R N
f : M →ₗ[R] N
inst_5 : Module.FinitePresentation R N
H :
  ∀ (I : Ideal R) (x : I.IsMaximal),
    ∃ (g :
      LinearMap (R := Localization.AtPrime I) (RingHom.id (Localization.AtPrime I)) (LocalizedModule I.primeCompl N)
        (LocalizedModule I.primeCompl M)),
      (LocalizedModule.map I.primeCompl :
              (a : M →ₗ[R] N) →
                LocalizedModule I.primeCompl M →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N)
            f ∘ₗ
          g =
        id
I : Ideal R
hI : I.IsMaximal
this :
  Membership.mem (γ :=
    Submodule (Localization (M := R) I.primeCompl)
      (LinearMap (R := Localization (M := R) I.primeCompl) (RingHom.id (Localization (M := R) I.primeCompl))
        (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl N)))
    (range (R := Localization (M := R) I.primeCompl) (M :=
      LinearMap (R := Localization (M := R) I.primeCompl) (RingHom.id (Localization.AtPrime I))
        (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl M))
      (τ₁₂ := RingHom.id (Localization (M := R) I.primeCompl))
      ((llcomp (Localization (M := R) I.primeCompl) (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl M)
            (LocalizedModule I.primeCompl N) :
          (a : LocalizedModule I.primeCompl M →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N) →
            (LocalizedModule I.primeCompl N →ₗ[Localization.AtPrime I]
                LocalizedModule I.primeCompl M) →ₗ[Localization I.primeCompl]
              LocalizedModule I.primeCompl N →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N)
        ((LocalizedModule.map I.primeCompl :
            (a : M →ₗ[R] N) →
              LocalizedModule I.primeCompl M →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N)
          f)))
    id
c : ↥I.primeCompl
g : N →ₗ[R] M
⊢ (((llcomp (Localization (M := R) I.primeCompl) (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl M)
                (LocalizedModule I.primeCompl N) :
              (a : LocalizedModule I.primeCompl M →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N) →
                (LocalizedModule I.primeCompl N →ₗ[Localization.AtPrime I]
                    LocalizedModule I.primeCompl M) →ₗ[Localization I.primeCompl]
                  LocalizedModule I.primeCompl N →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N)
            ((LocalizedModule.map I.primeCompl :
                (a : M →ₗ[R] N) →
                  LocalizedModule I.primeCompl M →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N)
              f) :
          LinearMap (R := Localization (M := R) I.primeCompl) (RingHom.id (Localization (M := R) I.primeCompl))
            (LinearMap (R := Localization (M := R) I.primeCompl) (RingHom.id (Localization.AtPrime I))
              (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl M))
            (LinearMap (R := Localization (M := R) I.primeCompl) (RingHom.id (Localization (M := R) I.primeCompl))
              (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl N))) :
        (a : LocalizedModule I.primeCompl N →ₗ[Localization.AtPrime I] LocalizedModule I.primeCompl M) →
          LocalizedModule I.primeCompl N →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N)
      ((↑c : R) • IsLocalizedModule.mk' (LocalizedModule.map I.primeCompl) g c) =
    HSMul.hSMul (β :=
      LinearMap (R := Localization (M := R) I.primeCompl) (RingHom.id (Localization (M := R) I.primeCompl))
        (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl N))
      c
      (IsLocalizedModule.mk' (LocalizedModule.map I.primeCompl)
        ((((llcomp R N M N : (a : M →ₗ[R] N) → (N →ₗ[R] M) →ₗ[R] N →ₗ[R] N) f : (N →ₗ[R] M) →ₗ[R] N →ₗ[R] N) :
            (a : N →ₗ[R] M) → N →ₗ[R] N)
          g)
        c)
have h : Eq (α :=
  LinearMap (R := Localization (M := R) I.primeCompl) (RingHom.id (Localization (M := R) I.primeCompl))
    (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl N))
  ((((llcomp (Localization (M := R) I.primeCompl) (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl M)
                (LocalizedModule I.primeCompl N) :
              (a : LocalizedModule I.primeCompl M →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N) →
                (LocalizedModule I.primeCompl N →ₗ[Localization.AtPrime I]
                    LocalizedModule I.primeCompl M) →ₗ[Localization I.primeCompl]
                  LocalizedModule I.primeCompl N →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N)
            ((LocalizedModule.map I.primeCompl :
                (a : M →ₗ[R] N) →
                  LocalizedModule I.primeCompl M →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N)
              f) :
          LinearMap (R := Localization (M := R) I.primeCompl) (RingHom.id (Localization (M := R) I.primeCompl))
            (LinearMap (R := Localization (M := R) I.primeCompl) (RingHom.id (Localization.AtPrime I))
              (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl M))
            (LinearMap (R := Localization (M := R) I.primeCompl) (RingHom.id (Localization (M := R) I.primeCompl))
              (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl N))) :
        (a : LocalizedModule I.primeCompl N →ₗ[Localization.AtPrime I] LocalizedModule I.primeCompl M) →
          LocalizedModule I.primeCompl N →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N)
      ((LocalizedModule.map I.primeCompl :
          (a : N →ₗ[R] M) → LocalizedModule I.primeCompl N →ₗ[Localization.AtPrime I] LocalizedModule I.primeCompl M)
        g) :
    LinearMap (R := Localization (M := R) I.primeCompl) (RingHom.id (Localization (M := R) I.primeCompl))
      (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl N))
  ((LocalizedModule.map I.primeCompl :
        (a : N →ₗ[R] N) → LocalizedModule I.primeCompl N →ₗ[Localization I.primeCompl] LocalizedModule I.primeCompl N)
      ((((llcomp R N M N : (a : M →ₗ[R] N) → (N →ₗ[R] M) →ₗ[R] N →ₗ[R] N) f : (N →ₗ[R] M) →ₗ[R] N →ₗ[R] N) :
          (a : N →ₗ[R] M) → N →ₗ[R] N)
        g) :
    LinearMap (R := Localization (M := R) I.primeCompl) (RingHom.id (Localization (M := R) I.primeCompl))
      (LocalizedModule I.primeCompl N) (LocalizedModule I.primeCompl N))
Tactic: rw [← Submonoid.smul_def, IsLocalizedModule.mk'_cancel', IsLocalizedModule.mk'_cancel']