-- 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 cons.cons.true
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
a : Γ'
L₁ : List Γ'
IH :
  ∀ {s : Option Γ'} {S : K' → List Γ'},
    splitAtPred p (S k) = (L₁, o, L₂) →
      Reaches₁ (TM2.step tr) { l := some (Λ'.clear p k q), var := s, stk := S }
        { l := some q, var := o, stk := update S k L₂ }
s : Option Γ'
S : K' → List Γ'
a' : Γ'
Sk : List Γ'
e₁ : S k = a' :: Sk
e₂ : p a' = true
e : ([], some a', Sk) = (a :: L₁, o, L₂)
⊢ 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 := o, stk := update S k L₂ }

Tactic:
cases e

Result:
no goals

Proof state:
case cons
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
a : Γ'
L₁ : List Γ'
IH :
  ∀ {s : Option Γ'} {S : K' → List Γ'},
    splitAtPred p (S k) = (L₁, o, L₂) →
      Reaches₁ (TM2.step tr) { l := some (Λ'.clear p k q), var := s, stk := S }
        { l := some q, var := o, stk := update S k L₂ }
s : Option Γ'
S : K' → List Γ'
e : splitAtPred p (S k) = (a :: L₁, o, L₂)
⊢ Reaches₁ (TM2.step tr) { l := some (Λ'.clear p k q), var := s, stk := S }
    { l := some q, var := o, stk := update S k L₂ }

Tactic:
refine TransGen.head rfl ?_

Result:
case cons
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
a : Γ'
L₁ : List Γ'
IH :
  ∀ {s : Option Γ'} {S : K' → List Γ'},
    splitAtPred p (S k) = (L₁, o, L₂) →
      Reaches₁ (TM2.step tr) { l := some (Λ'.clear p k q), var := s, stk := S }
        { l := some q, var := o, stk := update S k L₂ }
s : Option Γ'
S : K' → List Γ'
e : splitAtPred p (S k) = (a :: L₁, o, L₂)
⊢ TransGen (fun a b => b ∈ TM2.step tr a) (TM2.stepAux (tr (Λ'.clear p k q)) s S)
    { l := some q, var := o, stk := update S k L₂ }

Proof state:
case cons.cons
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
a : Γ'
L₁ : List Γ'
IH :
  ∀ {s : Option Γ'} {S : K' → List Γ'},
    splitAtPred p (S k) = (L₁, o, L₂) →
      Reaches₁ (TM2.step tr) { l := some (Λ'.clear p k q), var := s, stk := S }
        { l := some q, var := o, stk := update S k L₂ }
s : Option Γ'
S : K' → List Γ'
a' : Γ'
Sk : List Γ'
e :
  (bif p a' then ([], some a', Sk)
    else
      match splitAtPred p Sk with
      | (l₁, o, l₂) => (a' :: l₁, o, l₂)) =
    (a :: L₁, o, L₂)
e₁ : S k = a' :: Sk
⊢ 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 := o, stk := update S k L₂ }

Tactic:
cases e₂ : p a' <;> simp only [e₂, cond] at e

Result:
case cons.cons.false
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
a : Γ'
L₁ : List Γ'
IH :
  ∀ {s : Option Γ'} {S : K' → List Γ'},
    splitAtPred p (S k) = (L₁, o, L₂) →
      Reaches₁ (TM2.step tr) { l := some (Λ'.clear p k q), var := s, stk := S }
        { l := some q, var := o, stk := update S k L₂ }
s : Option Γ'
S : K' → List Γ'
a' : Γ'
Sk : List Γ'
e₁ : S k = 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 =>
      (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 }

case cons.cons.true
p : Γ' → Bool
k : K'
q : Λ'
o : Option Γ'
L₂ : List Γ'
a : Γ'
L₁ : List Γ'
IH :
  ∀ {s : Option Γ'} {S : K' → List Γ'},
    splitAtPred p (S k) = (L₁, o, L₂) →
      Reaches₁ (TM2.step tr) { l := some (Λ'.clear p k q), var := s, stk := S }
        { l := some q, var := o, stk := update S k L₂ }
s : Option Γ'
S : K' → List Γ'
a' : Γ'
Sk : List Γ'
e₁ : S k = a' :: Sk
e₂ : p a' = true
e : ([], some a', Sk) = (a :: L₁, o, L₂)
⊢ 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 := o, stk := 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)
⊢ Reaches₁ (TM2.step tr) { l := some (Λ'.clear p k q), var := s, stk := S }
    { l := some q, var := o, stk := update S k L₂ }

Tactic:
refine TransGen.head' rfl ?_

Result:
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 => b ∈ TM2.step tr a) (TM2.stepAux (tr (Λ'.clear p k q)) s S)
    { l := some q, var := o, stk := update S k L₂ }