-- Mathlib/Computability/TMToPartrec.lean
theorem move_ok {p k₁ k₂ q s L₁ o L₂} {S : K' → List Γ'} (h₁ : k₁ ≠ k₂)
    (e : splitAtPred p (S k₁) = (L₁, o, L₂)) :
    Reaches₁ (TM2.step tr) ⟨some (Λ'.move p k₁ k₂ q), s, S⟩
      ⟨some q, o, update (update S k₁ L₂) k₂ (L₁.reverseAux (S k₂))⟩

Proof state:
case cons.cons.false
p : Γ' → Bool
k₁ k₂ : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
h₁ : k₁ ≠ k₂
a : Γ'
L₁ : List Γ'
IH :
  ∀ {s : Option Γ'} {S : K' → List Γ'},
    splitAtPred p (S k₁) = (L₁, o, L₂) →
      Reaches₁ (TM2.step tr) { l := some (Λ'.move p k₁ k₂ q), var := s, stk := S }
        { l := some q, var := o, stk := update (update S k₁ L₂) k₂ (L₁.reverseAux (S k₂)) }
s : Option Γ'
S : K' → List Γ'
a' : Γ'
Sk : List Γ'
e₁ : S k₁ = a' :: Sk
e₂ : p a' = false
e : (a' :: (splitAtPred p Sk).1, (splitAtPred p Sk).2.1, (splitAtPred p Sk).2.2) = (a :: L₁, o, L₂)
⊢ TransGen (fun a b => b ∈ TM2.step tr a)
    (bif
        match (a' :: Sk).head?, true, p with
        | some x, x_1, f => f x
        | none, y, x => y then
      { l := some q, var := (a' :: Sk).head?, stk := update S k₁ (a' :: Sk).tail }
    else
      { l := some (Λ'.move p k₁ k₂ q), var := (a' :: Sk).head?,
        stk := update (update S k₁ (a' :: Sk).tail) k₂ ((a' :: Sk).head?.iget :: update S k₁ (a' :: Sk).tail k₂) })
    { l := some q, var := o, stk := update (update S k₁ L₂) k₂ ((a :: L₁).reverseAux (S k₂)) }

case cons.cons.true
p : Γ' → Bool
k₁ k₂ : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
h₁ : k₁ ≠ k₂
a : Γ'
L₁ : List Γ'
IH :
  ∀ {s : Option Γ'} {S : K' → List Γ'},
    splitAtPred p (S k₁) = (L₁, o, L₂) →
      Reaches₁ (TM2.step tr) { l := some (Λ'.move p k₁ k₂ q), var := s, stk := S }
        { l := some q, var := o, stk := update (update S k₁ L₂) k₂ (L₁.reverseAux (S k₂)) }
s : Option Γ'
S : K' → List Γ'
a' : Γ'
Sk : List Γ'
e₁ : S k₁ = a' :: Sk
e₂ : p a' = true
e : ([], some a', Sk) = (a :: L₁, o, L₂)
⊢ TransGen (fun a b => b ∈ TM2.step tr a)
    (bif
        match (a' :: Sk).head?, true, p with
        | some x, x_1, f => f x
        | none, y, x => y then
      { l := some q, var := (a' :: Sk).head?, stk := update S k₁ (a' :: Sk).tail }
    else
      { l := some (Λ'.move p k₁ k₂ q), var := (a' :: Sk).head?,
        stk := update (update S k₁ (a' :: Sk).tail) k₂ ((a' :: Sk).head?.iget :: update S k₁ (a' :: Sk).tail k₂) })
    { l := some q, var := o, stk := update (update S k₁ L₂) k₂ ((a :: L₁).reverseAux (S k₂)) }

Tactic:
swap

Result:
case cons.cons.true
p : Γ' → Bool
k₁ k₂ : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
h₁ : k₁ ≠ k₂
a : Γ'
L₁ : List Γ'
IH :
  ∀ {s : Option Γ'} {S : K' → List Γ'},
    splitAtPred p (S k₁) = (L₁, o, L₂) →
      Reaches₁ (TM2.step tr) { l := some (Λ'.move p k₁ k₂ q), var := s, stk := S }
        { l := some q, var := o, stk := update (update S k₁ L₂) k₂ (L₁.reverseAux (S k₂)) }
s : Option Γ'
S : K' → List Γ'
a' : Γ'
Sk : List Γ'
e₁ : S k₁ = a' :: Sk
e₂ : p a' = true
e : ([], some a', Sk) = (a :: L₁, o, L₂)
⊢ TransGen (fun a b => b ∈ TM2.step tr a)
    (bif
        match (a' :: Sk).head?, true, p with
        | some x, x_1, f => f x
        | none, y, x => y then
      { l := some q, var := (a' :: Sk).head?, stk := update S k₁ (a' :: Sk).tail }
    else
      { l := some (Λ'.move p k₁ k₂ q), var := (a' :: Sk).head?,
        stk := update (update S k₁ (a' :: Sk).tail) k₂ ((a' :: Sk).head?.iget :: update S k₁ (a' :: Sk).tail k₂) })
    { l := some q, var := o, stk := update (update S k₁ L₂) k₂ ((a :: L₁).reverseAux (S k₂)) }

case cons.cons.false
p : Γ' → Bool
k₁ k₂ : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
h₁ : k₁ ≠ k₂
a : Γ'
L₁ : List Γ'
IH :
  ∀ {s : Option Γ'} {S : K' → List Γ'},
    splitAtPred p (S k₁) = (L₁, o, L₂) →
      Reaches₁ (TM2.step tr) { l := some (Λ'.move p k₁ k₂ q), var := s, stk := S }
        { l := some q, var := o, stk := update (update S k₁ L₂) k₂ (L₁.reverseAux (S k₂)) }
s : Option Γ'
S : K' → List Γ'
a' : Γ'
Sk : List Γ'
e₁ : S k₁ = a' :: Sk
e₂ : p a' = false
e : (a' :: (splitAtPred p Sk).1, (splitAtPred p Sk).2.1, (splitAtPred p Sk).2.2) = (a :: L₁, o, L₂)
⊢ TransGen (fun a b => b ∈ TM2.step tr a)
    (bif
        match (a' :: Sk).head?, true, p with
        | some x, x_1, f => f x
        | none, y, x => y then
      { l := some q, var := (a' :: Sk).head?, stk := update S k₁ (a' :: Sk).tail }
    else
      { l := some (Λ'.move p k₁ k₂ q), var := (a' :: Sk).head?,
        stk := update (update S k₁ (a' :: Sk).tail) k₂ ((a' :: Sk).head?.iget :: update S k₁ (a' :: Sk).tail k₂) })
    { l := some q, var := o, stk := update (update S k₁ L₂) k₂ ((a :: L₁).reverseAux (S k₂)) }

Proof state:
case nil
p : Γ' → Bool
k₁ k₂ : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
h₁ : k₁ ≠ k₂
s : Option Γ'
S : K' → List Γ'
e : splitAtPred p (S k₁) = ([], o, L₂)
⊢ ReflTransGen (fun a b => b ∈ TM2.step tr a) (TM2.stepAux (tr (Λ'.move p k₁ k₂ q)) s S)
    { l := some q, var := o, stk := update S k₁ L₂ }

Tactic:
rw [tr]

Result:
case nil
p : Γ' → Bool
k₁ k₂ : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
h₁ : k₁ ≠ k₂
s : Option Γ'
S : K' → List Γ'
e : splitAtPred p (S k₁) = ([], o, L₂)
⊢ ReflTransGen (fun a b => b ∈ TM2.step tr a)
    (TM2.stepAux
      (pop' k₁ (branch (fun s => s.elim true p) (goto fun x => q) (push' k₂ (goto fun x => Λ'.move p k₁ k₂ q)))) s S)
    { l := some q, var := o, stk := update S k₁ L₂ }

Proof state:
p : Γ' → Bool
k₁ k₂ : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
h₁ : k₁ ≠ k₂
s : Option Γ'
S : K' → List Γ'
e : splitAtPred p (S k₁) = ([], o, L₂)
⊢ [].reverseAux (S k₂) = update S k₁ L₂ k₂

Tactic:
rw [Function.update_of_ne h₁.symm, List.reverseAux_nil]

Result:
no goals

Proof state:
case nil.cons
p : Γ' → Bool
k₁ k₂ : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
h₁ : k₁ ≠ k₂
s : Option Γ'
S : K' → List Γ'
a : Γ'
Sk : List Γ'
⊢ (bif p a then ([], some a, Sk) else (a :: (splitAtPred p Sk).1, (splitAtPred p Sk).2.1, (splitAtPred p Sk).2.2)) =
      ([], o, L₂) →
    ReflTransGen (fun a b => b ∈ TM2.step tr a)
      (bif p a then { l := some q, var := some a, stk := update S k₁ Sk }
      else TM2.stepAux (push' k₂ (goto fun x => Λ'.move p k₁ k₂ q)) (some a) (update S k₁ Sk))
      { l := some q, var := o, stk := update S k₁ L₂ }

Tactic:
cases p a <;> intro e <;>
simp only [cond_false, cond_true, Prod.mk.injEq, true_and, false_and, reduceCtorEq] at e ⊢

Result:
case nil.cons.true
p : Γ' → Bool
k₁ k₂ : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
h₁ : k₁ ≠ k₂
s : Option Γ'
S : K' → List Γ'
a : Γ'
Sk : List Γ'
e : some a = o ∧ Sk = L₂
⊢ ReflTransGen (fun a b => b ∈ TM2.step tr a) { l := some q, var := some a, stk := update S k₁ Sk }
    { l := some q, var := o, stk := update S k₁ L₂ }