-- Lean 4 proof step from Mathlib.RingTheory.MvPolynomial.Groebner (goal)
Goal:
σ : Type u_1
m : MonomialOrder.{u_1, u_4} σ
R : Type u_2
inst : CommRing R
ι : Type u_3
b : ι → MvPolynomial σ R
hb : ∀ (i : ι), IsUnit (m.leadingCoeff (b i))
f : MvPolynomial σ R
hb' : ∀ (i : ι), m.degree (b i) ≠ 0
hf0 : ¬f = 0
i : ι
hf : m.degree (b i) ≤ m.degree f
deg_reduce :
  LT.lt.{u_4} (α := m.syn) ((m.toSyn : (a : σ →₀ ℕ) → m.syn) (m.degree (m.reduce ⋯ f)) : m.syn)
    ((m.toSyn : (a : σ →₀ ℕ) → m.syn) (m.degree f) : m.syn)
g' : ι →₀ MvPolynomial σ R
r' : MvPolynomial σ R
H' :
  m.reduce ⋯ f =
      HAdd.hAdd (α := MvPolynomial σ R)
        ((Finsupp.linearCombination (MvPolynomial σ R) b : (a : ι →₀ MvPolynomial σ R) → MvPolynomial σ R) g' :
          MvPolynomial σ R)
        r' ∧
    (∀ (i_1 : ι),
        LE.le.{u_4} (α := m.syn)
          ((m.toSyn : (a : σ →₀ ℕ) → m.syn)
              (m.degree
                (HMul.hMul (β := MvPolynomial σ R) (b i_1)
                  ((g' : (a : ι) → MvPolynomial σ R) i_1 : MvPolynomial σ R))) :
            m.syn)
          ((m.toSyn : (a : σ →₀ ℕ) → m.syn) (m.degree (m.reduce ⋯ f)) : m.syn)) ∧
      ∀ c ∈ r'.support, ∀ (i : ι), ¬m.degree (b i) ≤ c
⊢ ∃ (g : ι →₀ MvPolynomial σ R) (r : MvPolynomial σ R),
    f =
        HAdd.hAdd (α := MvPolynomial σ R)
          ((Finsupp.linearCombination (MvPolynomial σ R) b : (a : ι →₀ MvPolynomial σ R) → MvPolynomial σ R) g :
            MvPolynomial σ R)
          r ∧
      (∀ (i : ι),
          LE.le.{u_4} (α := m.syn)
            ((m.toSyn : (a : σ →₀ ℕ) → m.syn)
                (m.degree
                  (HMul.hMul (β := MvPolynomial σ R) (b i) ((g : (a : ι) → MvPolynomial σ R) i : MvPolynomial σ R))) :
              m.syn)
            ((m.toSyn : (a : σ →₀ ℕ) → m.syn) (m.degree f) : m.syn)) ∧
        ∀ c ∈ r.support, ∀ (i : ι), ¬m.degree (b i) ≤ c
have h : ∃ (r : MvPolynomial σ R),
  f =
      HAdd.hAdd (α := MvPolynomial σ R)
        ((Finsupp.linearCombination (MvPolynomial σ R) b : (a : ι →₀ MvPolynomial σ R) → MvPolynomial σ R)
            (HAdd.hAdd (β := ι →₀ MvPolynomial σ R) g'
              (Finsupp.single i
                  ((monomial (m.degree f - m.degree (b i)) : (a : R) → MvPolynomial σ R)
                    ((↑sorry.unit⁻¹ : R) * m.leadingCoeff f)) :
                ι →₀ MvPolynomial σ R)) :
          MvPolynomial σ R)
        r ∧
    (∀ (i_1 : ι),
        LE.le.{u_4} (α := m.syn)
          ((m.toSyn : (a : σ →₀ ℕ) → m.syn)
              (m.degree
                (HMul.hMul (β := MvPolynomial σ R) (b i_1)
                  ((HAdd.hAdd (β := ι →₀ MvPolynomial σ R) g'
                          (Finsupp.single i
                              ((monomial (m.degree f - m.degree (b i)) : (a : R) → MvPolynomial σ R)
                                ((↑sorry.unit⁻¹ : R) * m.leadingCoeff f)) :
                            ι →₀ MvPolynomial σ R) :
                        (a : ι) → MvPolynomial σ R)
                      i_1 :
                    MvPolynomial σ R))) :
            m.syn)
          ((m.toSyn : (a : σ →₀ ℕ) → m.syn) (m.degree f) : m.syn)) ∧
      ∀ c ∈ r.support, ∀ (i : ι), ¬m.degree (b i) ≤ c
Tactic: use g' +
      Finsupp.single i (monomial (m.degree f - m.degree (b i)) ((hb i).unit⁻¹ * m.leadingCoeff f))