-- Mathlib/RingTheory/Regular/RegularSequence.lean
lemma IsWeaklyRegular.prototype_perm {rs : List R} (h : IsWeaklyRegular M rs)
    {rs'} (h'' : rs ~ rs') (h' : ∀ a b rs', (a :: b :: rs') <+~ rs →
      let K := torsionBy R (M ⧸ (Ideal.ofList rs' • ⊤ : Submodule R M)) b
      K = a • K → K = ⊥) : IsWeaklyRegular M rs' :=
  have H := LinearEquiv.isWeaklyRegular_congr <| quotEquivOfEqBot _ <|
    Eq.trans (congrArg (· • ⊤) Ideal.ofList_nil) (bot_smul ⊤)
  (H rs').mp <| (aux [] h'' (.refl rs) (h''.symm.subperm)) <| (H rs).mpr h
  where aux {rs₁ rs₂} (rs₀ : List R)
    (h₁₂ : rs₁ ~ rs₂) (H₁ : rs₀ ++ rs₁ <+~ rs) (H₃ : rs₀ ++ rs₂ <+~ rs)
    (h : IsWeaklyRegular (M ⧸ (Ideal.ofList rs₀ • ⊤ : Submodule R M)) rs₁) :
    IsWeaklyRegular (M ⧸ (Ideal.ofList rs₀ • ⊤ : Submodule R M)) rs₂

Proof state:
case cons
R : Type u_1
M : Type u_3
inst✝² : CommRing R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
rs : List R
h✝ : IsWeaklyRegular M rs
rs' : List R
h'' : rs ~ rs'
h' :
  ∀ (a b : R) (rs' : List R),
    a :: b :: rs' <+~ rs →
      let K := torsionBy R (M ⧸ Ideal.ofList rs' • ⊤) b;
      K = a • K → K = ⊥
rs₁ rs₂ : List R
r : R
l₁✝ l₂✝ : List R
a✝ : l₁✝ ~ l₂✝
ih :
  ∀ (rs₀ : List R),
    rs₀ ++ l₁✝ <+~ rs →
      rs₀ ++ l₂✝ <+~ rs →
        IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) l₁✝ → IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) l₂✝
rs₀ : List R
H₁ : rs₀ ++ r :: l₁✝ <+~ rs
H₃ : rs₀ ++ r :: l₂✝ <+~ rs
h : IsSMulRegular (M ⧸ Ideal.ofList rs₀ • ⊤) r ∧ IsWeaklyRegular (M ⧸ Ideal.ofList (r :: rs₀) • ⊤) l₁✝
e : (M ⧸ Ideal.ofList (r :: rs₀) • ⊤) ≃ₗ[R] QuotSMulTop r (M ⧸ Ideal.ofList rs₀ • ⊤) :=
  quotOfListConsSMulTopEquivQuotSMulTopOuter M r rs₀
⊢ IsSMulRegular (M ⧸ Ideal.ofList rs₀ • ⊤) r ∧ IsWeaklyRegular (M ⧸ Ideal.ofList (r :: rs₀) • ⊤) l₂✝

Tactic:
refine h.imp_right (ih (r :: rs₀) ?_ ?_) <;>
  exact List.perm_middle.subperm_right.mp ‹_›

Result:
no goals

Proof state:
case swap
R : Type u_1
M : Type u_3
inst✝² : CommRing R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
rs : List R
h✝ : IsWeaklyRegular M rs
rs' : List R
h'' : rs ~ rs'
h' :
  ∀ (a b : R) (rs' : List R),
    a :: b :: rs' <+~ rs →
      let K := torsionBy R (M ⧸ Ideal.ofList rs' • ⊤) b;
      K = a • K → K = ⊥
rs₁ rs₂ : List R
a b : R
t rs₀ : List R
H₁ : rs₀ ++ b :: a :: t <+~ rs
H₃ : rs₀ ++ a :: b :: t <+~ rs
h :
  IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) [b, a] ∧
    IsWeaklyRegular ((M ⧸ Ideal.ofList rs₀ • ⊤) ⧸ Ideal.ofList [a, b] • ⊤) t
this : Ideal.ofList [b, a] = Ideal.ofList [a, b]
⊢ IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) [a, b] ∧
    IsWeaklyRegular ((M ⧸ Ideal.ofList rs₀ • ⊤) ⧸ Ideal.ofList [a, b] • ⊤) t

Tactic:
rw [List.append_cons, List.append_cons, List.append_assoc _ [b] [a]] at H₁

Result:
case swap
R : Type u_1
M : Type u_3
inst✝² : CommRing R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
rs : List R
h✝ : IsWeaklyRegular M rs
rs' : List R
h'' : rs ~ rs'
h' :
  ∀ (a b : R) (rs' : List R),
    a :: b :: rs' <+~ rs →
      let K := torsionBy R (M ⧸ Ideal.ofList rs' • ⊤) b;
      K = a • K → K = ⊥
rs₁ rs₂ : List R
a b : R
t rs₀ : List R
H₁ : rs₀ ++ ([b] ++ [a]) ++ t <+~ rs
H₃ : rs₀ ++ a :: b :: t <+~ rs
h :
  IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) [b, a] ∧
    IsWeaklyRegular ((M ⧸ Ideal.ofList rs₀ • ⊤) ⧸ Ideal.ofList [a, b] • ⊤) t
this : Ideal.ofList [b, a] = Ideal.ofList [a, b]
⊢ IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) [a, b] ∧
    IsWeaklyRegular ((M ⧸ Ideal.ofList rs₀ • ⊤) ⧸ Ideal.ofList [a, b] • ⊤) t

Proof state:
case swap
R : Type u_1
M : Type u_3
inst✝² : CommRing R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
rs : List R
h✝ : IsWeaklyRegular M rs
rs' : List R
h'' : rs ~ rs'
h' :
  ∀ (a b : R) (rs' : List R),
    a :: b :: rs' <+~ rs →
      let K := torsionBy R (M ⧸ Ideal.ofList rs' • ⊤) b;
      K = a • K → K = ⊥
rs₁ rs₂ : List R
a b : R
t rs₀ : List R
H₁ : rs₀ ++ b :: a :: t <+~ rs
H₃ : rs₀ ++ a :: b :: t <+~ rs
h :
  IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) [b, a] ∧
    IsWeaklyRegular ((M ⧸ Ideal.ofList rs₀ • ⊤) ⧸ Ideal.ofList [b, a] • ⊤) t
⊢ IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) [a, b] ∧
    IsWeaklyRegular ((M ⧸ Ideal.ofList rs₀ • ⊤) ⧸ Ideal.ofList [a, b] • ⊤) t

Tactic:
have : Ideal.ofList [b, a] = Ideal.ofList [a, b] :=
  congrArg Ideal.span <| Set.ext fun _ => (List.Perm.swap a b []).mem_iff

Result:
case swap
R : Type u_1
M : Type u_3
inst✝² : CommRing R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
rs : List R
h✝ : IsWeaklyRegular M rs
rs' : List R
h'' : rs ~ rs'
h' :
  ∀ (a b : R) (rs' : List R),
    a :: b :: rs' <+~ rs →
      let K := torsionBy R (M ⧸ Ideal.ofList rs' • ⊤) b;
      K = a • K → K = ⊥
rs₁ rs₂ : List R
a b : R
t rs₀ : List R
H₁ : rs₀ ++ b :: a :: t <+~ rs
H₃ : rs₀ ++ a :: b :: t <+~ rs
h :
  IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) [b, a] ∧
    IsWeaklyRegular ((M ⧸ Ideal.ofList rs₀ • ⊤) ⧸ Ideal.ofList [b, a] • ⊤) t
this : Ideal.ofList [b, a] = Ideal.ofList [a, b]
⊢ IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) [a, b] ∧
    IsWeaklyRegular ((M ⧸ Ideal.ofList rs₀ • ⊤) ⧸ Ideal.ofList [a, b] • ⊤) t

Proof state:
case trans
R : Type u_1
M : Type u_3
inst✝² : CommRing R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
rs : List R
h✝ : IsWeaklyRegular M rs
rs' : List R
h'' : rs ~ rs'
h' :
  ∀ (a b : R) (rs' : List R),
    a :: b :: rs' <+~ rs →
      let K := torsionBy R (M ⧸ Ideal.ofList rs' • ⊤) b;
      K = a • K → K = ⊥
rs₁ rs₂ l₁✝ l₂✝ l₃✝ : List R
h₁₂ : l₁✝ ~ l₂✝
a✝ : l₂✝ ~ l₃✝
ih₁₂ :
  ∀ (rs₀ : List R),
    rs₀ ++ l₁✝ <+~ rs →
      rs₀ ++ l₂✝ <+~ rs →
        IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) l₁✝ → IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) l₂✝
ih₂₃ :
  ∀ (rs₀ : List R),
    rs₀ ++ l₂✝ <+~ rs →
      rs₀ ++ l₃✝ <+~ rs →
        IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) l₂✝ → IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) l₃✝
rs₀ : List R
H₁ : rs₀ ++ l₁✝ <+~ rs
H₃ : rs₀ ++ l₃✝ <+~ rs
h : IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) l₁✝
H₂ : rs₀ ++ l₂✝ <+~ rs
⊢ IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) l₃✝

Tactic:
exact ih₂₃ rs₀ H₂ H₃ (ih₁₂ rs₀ H₁ H₂ h)

Result:
no goals

Proof state:
case swap
R : Type u_1
M : Type u_3
inst✝² : CommRing R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
rs : List R
h✝ : IsWeaklyRegular M rs
rs' : List R
h'' : rs ~ rs'
h' :
  ∀ (a b : R) (rs' : List R),
    a :: b :: rs' <+~ rs →
      let K := torsionBy R (M ⧸ Ideal.ofList rs' • ⊤) b;
      K = a • K → K = ⊥
rs₁ rs₂ : List R
a b : R
t rs₀ : List R
H₁ : rs₀ ++ b :: a :: t <+~ rs
H₃ : rs₀ ++ a :: b :: t <+~ rs
h :
  IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) [b, a] ∧
    IsWeaklyRegular ((M ⧸ Ideal.ofList rs₀ • ⊤) ⧸ Ideal.ofList [b, a] • ⊤) t
this : Ideal.ofList [b, a] = Ideal.ofList [a, b]
⊢ IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) [a, b] ∧
    IsWeaklyRegular ((M ⧸ Ideal.ofList rs₀ • ⊤) ⧸ Ideal.ofList [a, b] • ⊤) t

Tactic:
rw [(quotEquivOfEq _ _ (congrArg₂ _ this rfl)).isWeaklyRegular_congr] at h

Result:
case swap
R : Type u_1
M : Type u_3
inst✝² : CommRing R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
rs : List R
h✝ : IsWeaklyRegular M rs
rs' : List R
h'' : rs ~ rs'
h' :
  ∀ (a b : R) (rs' : List R),
    a :: b :: rs' <+~ rs →
      let K := torsionBy R (M ⧸ Ideal.ofList rs' • ⊤) b;
      K = a • K → K = ⊥
rs₁ rs₂ : List R
a b : R
t rs₀ : List R
H₁ : rs₀ ++ b :: a :: t <+~ rs
H₃ : rs₀ ++ a :: b :: t <+~ rs
h :
  IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) [b, a] ∧
    IsWeaklyRegular ((M ⧸ Ideal.ofList rs₀ • ⊤) ⧸ Ideal.ofList [a, b] • ⊤) t
this : Ideal.ofList [b, a] = Ideal.ofList [a, b]
⊢ IsWeaklyRegular (M ⧸ Ideal.ofList rs₀ • ⊤) [a, b] ∧
    IsWeaklyRegular ((M ⧸ Ideal.ofList rs₀ • ⊤) ⧸ Ideal.ofList [a, b] • ⊤) t