Theorem: Polynomial.modByMonic_eq_sub_mul_div
File: Mathlib/Algebra/Polynomial/Div.lean

Proof steps:
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
⊢ p %ₘ q = p - q * (p /ₘ q)
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
⊢ p %ₘ q = p - q * (p /ₘ q)
tactic: have _wf := div_wf_lemma h hq
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
⊢ p %ₘ q = p - q * (p /ₘ q)
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) %ₘ q =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ p %ₘ q = p - q * (p /ₘ q)
tactic: have ih := modByMonic_eq_sub_mul_div
        (p - q * (C (leadingCoeff p) * X ^ (natDegree p - natDegree q))) hq
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) %ₘ q =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ p %ₘ q = p - q * (p /ₘ q)
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) %ₘ q =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ (if h : q.Monic then
      (if h_1 : q.degree ≤ p.degree ∧ p ≠ 0 then
          let z := C p.leadingCoeff * X ^ (p.natDegree - q.natDegree);
          let_fun _wf := ⋯;
          let dm := (p - q * z).divModByMonicAux ⋯;
          (z + dm.1, dm.2)
        else (0, p)).2
    else p) =
    p -
      q *
        if h : q.Monic then
          (if h_1 : q.degree ≤ p.degree ∧ p ≠ 0 then
              let z := C p.leadingCoeff * X ^ (p.natDegree - q.natDegree);
              let_fun _wf := ⋯;
              let dm := (p - q * z).divModByMonicAux ⋯;
              (z + dm.1, dm.2)
            else (0, p)).1
        else 0
tactic: unfold modByMonic divByMonic divModByMonicAux
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) %ₘ q =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ (if h : q.Monic then
      (if h_1 : q.degree ≤ p.degree ∧ p ≠ 0 then
          let z := C p.leadingCoeff * X ^ (p.natDegree - q.natDegree);
          let_fun _wf := ⋯;
          let dm := (p - q * z).divModByMonicAux ⋯;
          (z + dm.1, dm.2)
        else (0, p)).2
    else p) =
    p -
      q *
        if h : q.Monic then
          (if h_1 : q.degree ≤ p.degree ∧ p ≠ 0 then
              let z := C p.leadingCoeff * X ^ (p.natDegree - q.natDegree);
              let_fun _wf := ⋯;
              let dm := (p - q * z).divModByMonicAux ⋯;
              (z + dm.1, dm.2)
            else (0, p)).1
        else 0
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) %ₘ q =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ (if h : q.Monic then
      (if q.degree ≤ p.degree ∧ ¬p = 0 then
          (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
              ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
            ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
        else (0, p)).2
    else p) =
    p -
      q *
        if h : q.Monic then
          (if q.degree ≤ p.degree ∧ ¬p = 0 then
              (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
                  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
                ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
            else (0, p)).1
        else 0
tactic: dsimp
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) %ₘ q =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ (if h : q.Monic then
      (if q.degree ≤ p.degree ∧ ¬p = 0 then
          (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
              ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
            ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
        else (0, p)).2
    else p) =
    p -
      q *
        if h : q.Monic then
          (if q.degree ≤ p.degree ∧ ¬p = 0 then
              (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
                  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
                ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
            else (0, p)).1
        else 0
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) %ₘ q =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
          ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
        ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2).2 =
    p -
      q *
        if h : q.Monic then
          (if q.degree ≤ p.degree ∧ ¬p = 0 then
              (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
                  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
                ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
            else (0, p)).1
        else 0
tactic: rw [dif_pos hq, if_pos h]
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) %ₘ q =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
          ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
        ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2).2 =
    p -
      q *
        if h : q.Monic then
          (if q.degree ≤ p.degree ∧ ¬p = 0 then
              (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
                  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
                ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
            else (0, p)).1
        else 0
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux hq).2 =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
          ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
        ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2).2 =
    p -
      q *
        if h : q.Monic then
          (if q.degree ≤ p.degree ∧ ¬p = 0 then
              (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
                  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
                ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
            else (0, p)).1
        else 0
tactic: rw [modByMonic, dif_pos hq] at ih
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux hq).2 =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
          ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
        ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2).2 =
    p -
      q *
        if h : q.Monic then
          (if q.degree ≤ p.degree ∧ ¬p = 0 then
              (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
                  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
                ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
            else (0, p)).1
        else 0
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux hq).2 =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q) =
    p -
      q *
        if h : q.Monic then
          (if q.degree ≤ p.degree ∧ ¬p = 0 then
              (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
                  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
                ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
            else (0, p)).1
        else 0
tactic: refine ih.trans ?_
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux hq).2 =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q) =
    p -
      q *
        if h : q.Monic then
          (if q.degree ≤ p.degree ∧ ¬p = 0 then
              (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
                  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
                ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
            else (0, p)).1
        else 0
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux hq).2 =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q *
        if hq : q.Monic then ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux hq).1
        else 0) =
    p -
      q *
        if h : q.Monic then
          (if q.degree ≤ p.degree ∧ ¬p = 0 then
              (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
                  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
                ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
            else (0, p)).1
        else 0
tactic: unfold divByMonic
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : q.degree ≤ p.degree ∧ p ≠ 0
_wf : (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).degree < p.degree
ih :
  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux hq).2 =
    p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q * ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))) /ₘ q)
⊢ (p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree)) -
      q *
        if hq : q.Monic then ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux hq).1
        else 0) =
    p -
      q *
        if h : q.Monic then
          (if q.degree ≤ p.degree ∧ ¬p = 0 then
              (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
                  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
                ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
            else (0, p)).1
        else 0
state_after: no goals
tactic: rw [dif_pos hq, dif_pos hq, if_pos h, mul_add, sub_add_eq_sub_sub]
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : ¬(q.degree ≤ p.degree ∧ p ≠ 0)
⊢ p %ₘ q = p - q * (p /ₘ q)
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : ¬(q.degree ≤ p.degree ∧ p ≠ 0)
⊢ (if h : q.Monic then
      (if h_1 : q.degree ≤ p.degree ∧ p ≠ 0 then
          let z := C p.leadingCoeff * X ^ (p.natDegree - q.natDegree);
          let_fun _wf := ⋯;
          let dm := (p - q * z).divModByMonicAux ⋯;
          (z + dm.1, dm.2)
        else (0, p)).2
    else p) =
    p -
      q *
        if h : q.Monic then
          (if h_1 : q.degree ≤ p.degree ∧ p ≠ 0 then
              let z := C p.leadingCoeff * X ^ (p.natDegree - q.natDegree);
              let_fun _wf := ⋯;
              let dm := (p - q * z).divModByMonicAux ⋯;
              (z + dm.1, dm.2)
            else (0, p)).1
        else 0
tactic: unfold modByMonic divByMonic divModByMonicAux
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : ¬(q.degree ≤ p.degree ∧ p ≠ 0)
⊢ (if h : q.Monic then
      (if h_1 : q.degree ≤ p.degree ∧ p ≠ 0 then
          let z := C p.leadingCoeff * X ^ (p.natDegree - q.natDegree);
          let_fun _wf := ⋯;
          let dm := (p - q * z).divModByMonicAux ⋯;
          (z + dm.1, dm.2)
        else (0, p)).2
    else p) =
    p -
      q *
        if h : q.Monic then
          (if h_1 : q.degree ≤ p.degree ∧ p ≠ 0 then
              let z := C p.leadingCoeff * X ^ (p.natDegree - q.natDegree);
              let_fun _wf := ⋯;
              let dm := (p - q * z).divModByMonicAux ⋯;
              (z + dm.1, dm.2)
            else (0, p)).1
        else 0
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : ¬(q.degree ≤ p.degree ∧ p ≠ 0)
⊢ (if h : q.Monic then
      (if q.degree ≤ p.degree ∧ ¬p = 0 then
          (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
              ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
            ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
        else (0, p)).2
    else p) =
    p -
      q *
        if h : q.Monic then
          (if q.degree ≤ p.degree ∧ ¬p = 0 then
              (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
                  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
                ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
            else (0, p)).1
        else 0
tactic: dsimp
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p✝ q✝ p q : R[X]
hq : q.Monic
this : DecidableEq R := Classical.decEq R
h : ¬(q.degree ≤ p.degree ∧ p ≠ 0)
⊢ (if h : q.Monic then
      (if q.degree ≤ p.degree ∧ ¬p = 0 then
          (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
              ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
            ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
        else (0, p)).2
    else p) =
    p -
      q *
        if h : q.Monic then
          (if q.degree ≤ p.degree ∧ ¬p = 0 then
              (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree) +
                  ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).1,
                ((p - q * (C p.leadingCoeff * X ^ (p.natDegree - q.natDegree))).divModByMonicAux ⋯).2)
            else (0, p)).1
        else 0
state_after: no goals
tactic: rw [dif_pos hq, if_neg h, dif_pos hq, if_neg h, mul_zero, sub_zero]