-- Mathlib/RingTheory/MvPolynomial/Groebner.lean
theorem div {ι : Type*} {b : ι → MvPolynomial σ R}
    (hb : ∀ i, IsUnit (m.leadingCoeff (b i))) (f : MvPolynomial σ R) :
    ∃ (g : ι →₀ (MvPolynomial σ R)) (r : MvPolynomial σ R),
      f = Finsupp.linearCombination _ b g + r ∧
        (∀ i, m.degree (b i * (g i)) ≼[m] m.degree f) ∧
        (∀ c ∈ r.support, ∀ i, ¬ (m.degree (b i) ≤ c))

Proof state:
case h.right
σ : Type u_1
m : MonomialOrder 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
hf : ∀ (i : ι), ¬m.degree (b i) ≤ m.degree f
g' : ι →₀ MvPolynomial σ R
r' : MvPolynomial σ R
H' :
  m.subLTerm f = (Finsupp.linearCombination (MvPolynomial σ R) b) g' + r' ∧
    (∀ (i : ι), m.toSyn (m.degree (b i * g' i)) ≤ m.toSyn (m.degree (m.subLTerm f))) ∧
      ∀ c ∈ r'.support, ∀ (i : ι), ¬m.degree (b i) ≤ c
⊢ (∀ (i : ι), m.toSyn (m.degree (b i * g' i)) ≤ m.toSyn (m.degree f)) ∧
    ∀ c ∈ (r' + (monomial (m.degree f)) (m.leadingCoeff f)).support, ∀ (i : ι), ¬m.degree (b i) ≤ c

Tactic:
constructor

Result:
case h.right.left
σ : Type u_1
m : MonomialOrder 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
hf : ∀ (i : ι), ¬m.degree (b i) ≤ m.degree f
g' : ι →₀ MvPolynomial σ R
r' : MvPolynomial σ R
H' :
  m.subLTerm f = (Finsupp.linearCombination (MvPolynomial σ R) b) g' + r' ∧
    (∀ (i : ι), m.toSyn (m.degree (b i * g' i)) ≤ m.toSyn (m.degree (m.subLTerm f))) ∧
      ∀ c ∈ r'.support, ∀ (i : ι), ¬m.degree (b i) ≤ c
⊢ ∀ (i : ι), m.toSyn (m.degree (b i * g' i)) ≤ m.toSyn (m.degree f)

case h.right.right
σ : Type u_1
m : MonomialOrder 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
hf : ∀ (i : ι), ¬m.degree (b i) ≤ m.degree f
g' : ι →₀ MvPolynomial σ R
r' : MvPolynomial σ R
H' :
  m.subLTerm f = (Finsupp.linearCombination (MvPolynomial σ R) b) g' + r' ∧
    (∀ (i : ι), m.toSyn (m.degree (b i * g' i)) ≤ m.toSyn (m.degree (m.subLTerm f))) ∧
      ∀ c ∈ r'.support, ∀ (i : ι), ¬m.degree (b i) ≤ c
⊢ ∀ c ∈ (r' + (monomial (m.degree f)) (m.leadingCoeff f)).support, ∀ (i : ι), ¬m.degree (b i) ≤ c

Proof state:
case neg
σ : Type u_1
m : MonomialOrder 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
hf : ¬∃ i, m.degree (b i) ≤ m.degree f
⊢ ∃ g r,
    f = (Finsupp.linearCombination (MvPolynomial σ R) b) g + r ∧
      (∀ (i : ι), m.toSyn (m.degree (b i * g i)) ≤ m.toSyn (m.degree f)) ∧
        ∀ c ∈ r.support, ∀ (i : ι), ¬m.degree (b i) ≤ c

Tactic:
push_neg at hf

Result:
case neg
σ : Type u_1
m : MonomialOrder 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
hf : ∀ (i : ι), ¬m.degree (b i) ≤ m.degree f
⊢ ∃ g r,
    f = (Finsupp.linearCombination (MvPolynomial σ R) b) g + r ∧
      (∀ (i : ι), m.toSyn (m.degree (b i * g i)) ≤ m.toSyn (m.degree f)) ∧
        ∀ c ∈ r.support, ∀ (i : ι), ¬m.degree (b i) ≤ c

Proof state:
case hf
σ : Type u_1
m : MonomialOrder 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
a✝ :
  ∀ (y : MvPolynomial σ R),
    (invImage (fun x => ⋯.wrap (m.toSyn (m.degree x))) Acc.wfRel).1 y f →
      ∃ g r,
        y = (Finsupp.linearCombination (MvPolynomial σ R) b) g + r ∧
          (∀ (i : ι), m.toSyn (m.degree (b i * g i)) ≤ m.toSyn (m.degree y)) ∧
            ∀ c ∈ r.support, ∀ (i : ι), ¬m.degree (b i) ≤ c
hb' : ¬∃ i, m.degree (b i) = 0
hf0✝ : ¬f = 0
hf : ¬∃ i, m.degree (b i) ≤ m.degree f
hf'0 : ¬m.subLTerm f = 0
hf0 : m.degree f = 0
⊢ False

Tactic:
apply hf'0

Result:
case hf
σ : Type u_1
m : MonomialOrder 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
a✝ :
  ∀ (y : MvPolynomial σ R),
    (invImage (fun x => ⋯.wrap (m.toSyn (m.degree x))) Acc.wfRel).1 y f →
      ∃ g r,
        y = (Finsupp.linearCombination (MvPolynomial σ R) b) g + r ∧
          (∀ (i : ι), m.toSyn (m.degree (b i * g i)) ≤ m.toSyn (m.degree y)) ∧
            ∀ c ∈ r.support, ∀ (i : ι), ¬m.degree (b i) ≤ c
hb' : ¬∃ i, m.degree (b i) = 0
hf0✝ : ¬f = 0
hf : ¬∃ i, m.degree (b i) ≤ m.degree f
hf'0 : ¬m.subLTerm f = 0
hf0 : m.degree f = 0
⊢ m.subLTerm f = 0

Proof state:
case h
σ : Type u_1
m : MonomialOrder 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 : m.toSyn (m.degree (m.reduce ⋯ f)) < m.toSyn (m.degree f)
g' : ι →₀ MvPolynomial σ R
r' : MvPolynomial σ R
H' :
  m.reduce ⋯ f = (Finsupp.linearCombination (MvPolynomial σ R) b) g' + r' ∧
    (∀ (i_1 : ι), m.toSyn (m.degree (b i_1 * g' i_1)) ≤ m.toSyn (m.degree (m.reduce ⋯ f))) ∧
      ∀ c ∈ r'.support, ∀ (i : ι), ¬m.degree (b i) ≤ c
⊢ ∃ r,
    f =
        (Finsupp.linearCombination (MvPolynomial σ R) b)
            (g' + Finsupp.single i ((monomial (m.degree f - m.degree (b i))) (↑⋯.unit⁻¹ * m.leadingCoeff f))) +
          r ∧
      (∀ (i_1 : ι),
          m.toSyn
              (m.degree
                (b i_1 *
                  (g' + Finsupp.single i ((monomial (m.degree f - m.degree (b i))) (↑⋯.unit⁻¹ * m.leadingCoeff f)))
                    i_1)) ≤
            m.toSyn (m.degree f)) ∧
        ∀ c ∈ r.support, ∀ (i : ι), ¬m.degree (b i) ≤ c

Tactic:
use r'

Result:
case h
σ : Type u_1
m : MonomialOrder 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 : m.toSyn (m.degree (m.reduce ⋯ f)) < m.toSyn (m.degree f)
g' : ι →₀ MvPolynomial σ R
r' : MvPolynomial σ R
H' :
  m.reduce ⋯ f = (Finsupp.linearCombination (MvPolynomial σ R) b) g' + r' ∧
    (∀ (i_1 : ι), m.toSyn (m.degree (b i_1 * g' i_1)) ≤ m.toSyn (m.degree (m.reduce ⋯ f))) ∧
      ∀ c ∈ r'.support, ∀ (i : ι), ¬m.degree (b i) ≤ c
⊢ f =
      (Finsupp.linearCombination (MvPolynomial σ R) b)
          (g' + Finsupp.single i ((monomial (m.degree f - m.degree (b i))) (↑⋯.unit⁻¹ * m.leadingCoeff f))) +
        r' ∧
    (∀ (i_1 : ι),
        m.toSyn
            (m.degree
              (b i_1 *
                (g' + Finsupp.single i ((monomial (m.degree f - m.degree (b i))) (↑⋯.unit⁻¹ * m.leadingCoeff f)))
                  i_1)) ≤
          m.toSyn (m.degree f)) ∧
      ∀ c ∈ r'.support, ∀ (i : ι), ¬m.degree (b i) ≤ c

Proof state:
case h.left
σ : Type u_1
m : MonomialOrder 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
i : ι
hb0 : m.degree (b i) = 0
⊢ m.leadingCoeff (b i) • f = f * b i

Tactic:
nth_rewrite 2 [eq_C_of_degree_eq_zero hb0]

Result:
case h.left
σ : Type u_1
m : MonomialOrder 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
i : ι
hb0 : m.degree (b i) = 0
⊢ m.leadingCoeff (b i) • f = f * C (m.leadingCoeff (b i))