-- Lean 4 proof step from Mathlib.RingTheory.MvPolynomial.Localization (goal)
Goal:
R : Type u_2
inst : CommRing R
S : Type u_3
inst_1 : CommRing S
inst_2 : Algebra R S
r : R
inst_3 : Away r S
⊢ Eq (α :=
    MvPolynomial Unit R →+*
      HasQuotient.Quotient (MvPolynomial Unit R) (B := Ideal (MvPolynomial Unit R))
        (Ideal.span
          {HMul.hMul (α := MvPolynomial Unit R) (β := MvPolynomial Unit R)
                ((C : (a : R) → MvPolynomial Unit R) r : MvPolynomial Unit R) (X ()) -
              1}))
    (RingHom.comp (@RingHom.comp _ _ _ _ Semiring.toNonAssocSemiring _ (auxInv S r) (AlgHom.toRingHom (auxHom S r)))
      (Ideal.Quotient.mk
        (Ideal.span
          {HMul.hMul (α := MvPolynomial Unit R) (β := MvPolynomial Unit R)
                ((C : (a : R) → MvPolynomial Unit R) r : MvPolynomial Unit R) (X ()) -
              1})))
    (RingHom.comp
      (RingHom.id
        (HasQuotient.Quotient (MvPolynomial Unit R) (B := Ideal (MvPolynomial Unit R))
          (Ideal.span
            {HMul.hMul (α := MvPolynomial Unit R) (β := MvPolynomial Unit R)
                  ((C : (a : R) → MvPolynomial Unit R) r : MvPolynomial Unit R) (X ()) -
                1})))
      (Ideal.Quotient.mk
        (Ideal.span
          {HMul.hMul (α := MvPolynomial Unit R) (β := MvPolynomial Unit R)
                ((C : (a : R) → MvPolynomial Unit R) r : MvPolynomial Unit R) (X ()) -
              1})))
have hC : ∀ (x : R),
  Eq (α :=
    HasQuotient.Quotient (MvPolynomial Unit R) (B := Ideal (MvPolynomial Unit R))
      (Ideal.span
        {HMul.hMul (α := MvPolynomial Unit R) (β := MvPolynomial Unit R)
              ((C : (a : R) → MvPolynomial Unit R) r : MvPolynomial Unit R) (X ()) -
            1}))
    ((RingHom.comp
            (RingHom.comp
              (@RingHom.comp _ _ _ _ Semiring.toNonAssocSemiring _ (auxInv S r) (AlgHom.toRingHom (auxHom S r)))
              (Ideal.Quotient.mk
                (Ideal.span
                  {HMul.hMul (α := MvPolynomial Unit R) (β := MvPolynomial Unit R)
                        ((C : (a : R) → MvPolynomial Unit R) r : MvPolynomial Unit R) (X ()) -
                      1})))
            C :
          (a : R) → MvPolynomial Unit R ⧸ Ideal.span {(C : (a : R) → MvPolynomial Unit R) r * X () - 1})
        x :
      HasQuotient.Quotient (MvPolynomial Unit R) (B := Ideal (MvPolynomial Unit R))
        (Ideal.span
          {HMul.hMul (α := MvPolynomial Unit R) (β := MvPolynomial Unit R)
                ((C : (a : R) → MvPolynomial Unit R) r : MvPolynomial Unit R) (X ()) -
              1}))
    ((RingHom.comp
            (RingHom.comp
              (RingHom.id
                (HasQuotient.Quotient (MvPolynomial Unit R) (B := Ideal (MvPolynomial Unit R))
                  (Ideal.span
                    {HMul.hMul (α := MvPolynomial Unit R) (β := MvPolynomial Unit R)
                          ((C : (a : R) → MvPolynomial Unit R) r : MvPolynomial Unit R) (X ()) -
                        1})))
              (Ideal.Quotient.mk
                (Ideal.span
                  {HMul.hMul (α := MvPolynomial Unit R) (β := MvPolynomial Unit R)
                        ((C : (a : R) → MvPolynomial Unit R) r : MvPolynomial Unit R) (X ()) -
                      1})))
            C :
          (a : R) → MvPolynomial Unit R ⧸ Ideal.span {(C : (a : R) → MvPolynomial Unit R) r * X () - 1})
        x :
      HasQuotient.Quotient (MvPolynomial Unit R) (B := Ideal (MvPolynomial Unit R))
        (Ideal.span
          {HMul.hMul (α := MvPolynomial Unit R) (β := MvPolynomial Unit R)
                ((C : (a : R) → MvPolynomial Unit R) r : MvPolynomial Unit R) (X ()) -
              1}))
Tactic: ext x