-- Mathlib/Computability/TMToPartrec.lean
theorem clear_ok {p k q s L₁ o L₂} {S : K' → List Γ'} (e : splitAtPred p (S k) = (L₁, o, L₂)) :
    Reaches₁ (TM2.step tr) ⟨some (Λ'.clear p k q), s, S⟩ ⟨some q, o, update S k L₂⟩

Proof state:
case nil
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
s : Option Γ'
S : K' → List Γ'
e : splitAtPred p (S k) = ([], o, L₂)
⊢ ReflTransGen
    (fun a b =>
      (match a with
        | { l := none, var := var, stk := stk } => none
        | { l := some l, var := v, stk := S } => some (TM2.stepAux (tr l) v S)) =
        some b)
    (bif
        match (S k).head?, true, p with
        | some x, x_1, f => f x
        | none, y, x => y then
      { l := some q, var := (S k).head?, stk := update S k (S k).tail }
    else { l := some (Λ'.clear p k q), var := (S k).head?, stk := update S k (S k).tail })
    { l := some q, var := o, stk := update S k L₂ }

Tactic:
revert e

Result:
case nil
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
s : Option Γ'
S : K' → List Γ'
⊢ splitAtPred p (S k) = ([], o, L₂) →
    ReflTransGen
      (fun a b =>
        (match a with
          | { l := none, var := var, stk := stk } => none
          | { l := some l, var := v, stk := S } => some (TM2.stepAux (tr l) v S)) =
          some b)
      (bif
          match (S k).head?, true, p with
          | some x, x_1, f => f x
          | none, y, x => y then
        { l := some q, var := (S k).head?, stk := update S k (S k).tail }
      else { l := some (Λ'.clear p k q), var := (S k).head?, stk := update S k (S k).tail })
      { l := some q, var := o, stk := update S k L₂ }

Proof state:
case nil.cons
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
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 =>
        (match a with
          | { l := none, var := var, stk := stk } => none
          | { l := some l, var := v, stk := S } => some (TM2.stepAux (tr l) v S)) =
          some b)
      (bif p a then { l := some q, var := some a, stk := update S k Sk }
      else { l := some (Λ'.clear p k q), var := some a, stk := update S k Sk })
      { l := some q, var := o, stk := update S k 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
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
s : Option Γ'
S : K' → List Γ'
a : Γ'
Sk : List Γ'
e : some a = o ∧ Sk = L₂
⊢ ReflTransGen
    (fun a b =>
      (match a with
        | { l := none, var := var, stk := stk } => none
        | { l := some l, var := v, stk := S } => some (TM2.stepAux (tr l) v S)) =
        some b)
    { l := some q, var := some a, stk := update S k Sk } { l := some q, var := o, stk := update S k L₂ }

Proof state:
case nil.cons
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
s : Option Γ'
S : K' → List Γ'
a : Γ'
Sk : List Γ'
e :
  (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 =>
      (match a with
        | { l := none, var := var, stk := stk } => none
        | { l := some l, var := v, stk := S } => some (TM2.stepAux (tr l) v S)) =
        some b)
    (bif p a then { l := some q, var := some a, stk := update S k Sk }
    else { l := some (Λ'.clear p k q), var := some a, stk := update S k Sk })
    { l := some q, var := o, stk := update S k L₂ }

Tactic:
revert e

Result:
case nil.cons
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
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 =>
        (match a with
          | { l := none, var := var, stk := stk } => none
          | { l := some l, var := v, stk := S } => some (TM2.stepAux (tr l) v S)) =
          some b)
      (bif p a then { l := some q, var := some a, stk := update S k Sk }
      else { l := some (Λ'.clear p k q), var := some a, stk := update S k Sk })
      { l := some q, var := o, stk := update S k L₂ }

Proof state:
case cons.cons.false.mk.mk.refl
p : Γ' → Bool
k : K'
q : Λ'
a : Γ'
s : Option Γ'
S : K' → List Γ'
Sk fst✝¹ : List Γ'
fst✝ : Option Γ'
snd✝ : List Γ'
e₃ : splitAtPred p Sk = (fst✝¹, fst✝, snd✝)
e₁ : S k = a :: Sk
e₂ : p a = false
IH :
  ∀ {s : Option Γ'} {S : K' → List Γ'},
    splitAtPred p (S k) = ((fst✝¹, fst✝, snd✝).1, (fst✝¹, fst✝, snd✝).2.1, (fst✝¹, fst✝, snd✝).2.2) →
      Reaches₁ (TM2.step tr) { l := some (Λ'.clear p k q), var := s, stk := S }
        { l := some q, var := (fst✝¹, fst✝, snd✝).2.1, stk := update S k (fst✝¹, fst✝, snd✝).2.2 }
⊢ TransGen
    (fun a b =>
      (match a with
        | { l := none, var := var, stk := stk } => none
        | { l := some l, var := v, stk := S } => some (TM2.stepAux (tr l) v S)) =
        some b)
    (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 k (a :: Sk).tail }
    else { l := some (Λ'.clear p k q), var := (a :: Sk).head?, stk := update S k (a :: Sk).tail })
    { l := some q, var := (fst✝¹, fst✝, snd✝).2.1, stk := update S k (fst✝¹, fst✝, snd✝).2.2 }

Tactic:
simp only [List.head?_cons, e₂, List.tail_cons, cond_false]

Result:
case cons.cons.false.mk.mk.refl
p : Γ' → Bool
k : K'
q : Λ'
a : Γ'
s : Option Γ'
S : K' → List Γ'
Sk fst✝¹ : List Γ'
fst✝ : Option Γ'
snd✝ : List Γ'
e₃ : splitAtPred p Sk = (fst✝¹, fst✝, snd✝)
e₁ : S k = a :: Sk
e₂ : p a = false
IH :
  ∀ {s : Option Γ'} {S : K' → List Γ'},
    splitAtPred p (S k) = ((fst✝¹, fst✝, snd✝).1, (fst✝¹, fst✝, snd✝).2.1, (fst✝¹, fst✝, snd✝).2.2) →
      Reaches₁ (TM2.step tr) { l := some (Λ'.clear p k q), var := s, stk := S }
        { l := some q, var := (fst✝¹, fst✝, snd✝).2.1, stk := update S k (fst✝¹, fst✝, snd✝).2.2 }
⊢ TransGen
    (fun a b =>
      (match a with
        | { l := none, var := var, stk := stk } => none
        | { l := some l, var := v, stk := S } => some (TM2.stepAux (tr l) v S)) =
        some b)
    { l := some (Λ'.clear p k q), var := some a, stk := update S k Sk }
    { l := some q, var := fst✝, stk := update S k snd✝ }

Proof state:
case nil.cons
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
s : Option Γ'
S : K' → List Γ'
a : Γ'
Sk : List Γ'
e : splitAtPred p (a :: Sk) = ([], o, L₂)
⊢ ReflTransGen
    (fun a b =>
      (match a with
        | { l := none, var := var, stk := stk } => none
        | { l := some l, var := v, stk := S } => some (TM2.stepAux (tr l) v S)) =
        some b)
    (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 k (a :: Sk).tail }
    else { l := some (Λ'.clear p k q), var := (a :: Sk).head?, stk := update S k (a :: Sk).tail })
    { l := some q, var := o, stk := update S k L₂ }

Tactic:
simp only [splitAtPred, Option.elim, List.head?, List.tail_cons] at e ⊢

Result:
case nil.cons
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
s : Option Γ'
S : K' → List Γ'
a : Γ'
Sk : List Γ'
e :
  (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 =>
      (match a with
        | { l := none, var := var, stk := stk } => none
        | { l := some l, var := v, stk := S } => some (TM2.stepAux (tr l) v S)) =
        some b)
    (bif p a then { l := some q, var := some a, stk := update S k Sk }
    else { l := some (Λ'.clear p k q), var := some a, stk := update S k Sk })
    { l := some q, var := o, stk := update S k L₂ }