-- Lean 4 proof step from Mathlib.RingTheory.Etale.StandardEtale (goal)
Goal:
R : Type u_1
S : Type u_2
T : Type u_3
inst : CommRing R
inst_1 : CommRing S
inst_2 : CommRing T
inst_3 : Algebra R S
inst_4 : Algebra R T
P : StandardEtalePair R
⊢ IsUnit (M :=
    HasQuotient.Quotient (Localization.Away P.g) (B := Ideal (Localization.Away P.g))
      (Ideal.span
        (singleton (α := Localization.Away P.g)
          ((algebraMap R[X] (Localization.Away P.g) : (a : R[X]) → Localization.Away P.g) P.f :
            Localization.Away P.g))))
    (((aeval
              ((algebraMap R[X]
                    (HasQuotient.Quotient (Localization.Away P.g) (B := Ideal (Localization.Away P.g))
                      (Ideal.span
                        (singleton (α := Localization.Away P.g)
                          ((algebraMap R[X] (Localization.Away P.g) : (a : R[X]) → Localization.Away P.g) P.f :
                            Localization.Away P.g)))) :
                  (a : R[X]) →
                    Localization.Away P.g ⧸
                      Ideal.span {(algebraMap R[X] (Localization.Away P.g) : (a : R[X]) → Localization.Away P.g) P.f})
                X) :
            R[X] →ₐ[R]
              HasQuotient.Quotient (Localization.Away P.g) (B := Ideal (Localization.Away P.g))
                (Ideal.span
                  (singleton (α := Localization.Away P.g)
                    ((algebraMap R[X] (Localization.Away P.g) : (a : R[X]) → Localization.Away P.g) P.f :
                      Localization.Away P.g)))) :
          (a : R[X]) →
            Localization.Away P.g ⧸
              Ideal.span {(algebraMap R[X] (Localization.Away P.g) : (a : R[X]) → Localization.Away P.g) P.f})
        P.g :
      HasQuotient.Quotient (Localization.Away P.g) (B := Ideal (Localization.Away P.g))
        (Ideal.span
          (singleton (α := Localization.Away P.g)
            ((algebraMap R[X] (Localization.Away P.g) : (a : R[X]) → Localization.Away P.g) P.f :
              Localization.Away P.g))))
have refine_2 : IsUnit (M :=
  HasQuotient.Quotient (Localization.Away P.g) (B := Ideal (Localization.Away P.g))
    (Ideal.span
      (singleton (α := Localization.Away P.g)
        ((algebraMap R[X] (Localization.Away P.g) : (a : R[X]) → Localization.Away P.g) P.f : Localization.Away P.g))))
  ((algebraMap (Localization.Away P.g)
          (HasQuotient.Quotient (Localization.Away P.g) (B := Ideal (Localization.Away P.g))
            (Ideal.span
              (singleton (α := Localization.Away P.g)
                ((algebraMap R[X] (Localization.Away P.g) : (a : R[X]) → Localization.Away P.g) P.f :
                  Localization.Away P.g)))) :
        (a : Localization.Away P.g) →
          Localization.Away P.g ⧸
            Ideal.span {(algebraMap R[X] (Localization.Away P.g) : (a : R[X]) → Localization.Away P.g) P.f})
      ((algebraMap R[X] (Localization.Away P.g) : (a : R[X]) → Localization.Away P.g) P.g) :
    HasQuotient.Quotient (Localization.Away P.g) (B := Ideal (Localization.Away P.g))
      (Ideal.span
        (singleton (α := Localization.Away P.g)
          ((algebraMap R[X] (Localization.Away P.g) : (a : R[X]) → Localization.Away P.g) P.f :
            Localization.Away P.g))))
Tactic: rw [aeval_algebraMap_apply, IsScalarTower.algebraMap_apply _ (Localization.Away P.g) (_ ⧸ _),
      aeval_X_left_apply]