-- 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
x : Unit
⊢ (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}) :
        (a : MvPolynomial Unit R) → MvPolynomial Unit R ⧸ Ideal.span {(C : (a : R) → MvPolynomial Unit R) r * X () - 1})
      ((C : (a : R) → MvPolynomial Unit R) 1) =
    HMul.hMul (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}))
      (β :=
      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}) :
            (a : MvPolynomial Unit R) →
              MvPolynomial Unit R ⧸ Ideal.span {(C : (a : R) → MvPolynomial Unit R) r * X () - 1})
          ((C : (a : R) → MvPolynomial Unit R) 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}))
      ((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}) :
            (a : MvPolynomial Unit R) →
              MvPolynomial Unit R ⧸ Ideal.span {(C : (a : R) → MvPolynomial Unit R) r * X () - 1})
          (X 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}))
have hX : 1 =
  HMul.hMul (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}))
    (β :=
    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}) :
          (a : MvPolynomial Unit R) →
            MvPolynomial Unit R ⧸ Ideal.span {(C : (a : R) → MvPolynomial Unit R) r * X () - 1})
        ((C : (a : R) → MvPolynomial Unit R) 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}))
    ((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}) :
          (a : MvPolynomial Unit R) →
            MvPolynomial Unit R ⧸ Ideal.span {(C : (a : R) → MvPolynomial Unit R) r * X () - 1})
        (X 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: simp only [map_one]