-- Mathlib/Computability/TMToPartrec.lean
theorem pred_ok (q₁ q₂ s v) (c d : List Γ') : ∃ s',
    Reaches₁ (TM2.step tr) ⟨some (Λ'.pred q₁ q₂), s, K'.elim (trList v) [] c d⟩
      (v.headI.rec ⟨some q₁, s', K'.elim (trList v.tail) [] c d⟩ fun n _ =>
        ⟨some q₂, s', K'.elim (trList (n::v.tail)) [] c d⟩)

Proof state:
case cons.succ.pos
q₁ q₂ : Λ'
s : Option Γ'
c d : List Γ'
v : List ℕ
n : ℕ
a : PosNum
⊢ ∀ (l₁ : List Γ'),
    ∃ l₁' l₂' s',
      l₁.reverseAux (trPosNum a) = l₁'.reverseAux l₂' ∧
        Reaches₁ (TM2.step tr)
          { l := some (q₁.pred q₂), var := s, stk := elim (trPosNum a.succ ++ Γ'.cons :: trList v) l₁ c d }
          { l := some (unrev q₂), var := s', stk := elim (l₂' ++ Γ'.cons :: trList v) l₁' c d }

Tactic:
induction' a with m IH m IH generalizing s <;> intro l₁

Result:
case cons.succ.pos.one
q₁ q₂ : Λ'
c d : List Γ'
v : List ℕ
n : ℕ
s : Option Γ'
l₁ : List Γ'
⊢ ∃ l₁' l₂' s',
    l₁.reverseAux (trPosNum PosNum.one) = l₁'.reverseAux l₂' ∧
      Reaches₁ (TM2.step tr)
        { l := some (q₁.pred q₂), var := s, stk := elim (trPosNum PosNum.one.succ ++ Γ'.cons :: trList v) l₁ c d }
        { l := some (unrev q₂), var := s', stk := elim (l₂' ++ Γ'.cons :: trList v) l₁' c d }

case cons.succ.pos.bit1
q₁ q₂ : Λ'
c d : List Γ'
v : List ℕ
n : ℕ
m : PosNum
IH :
  ∀ (s : Option Γ') (l₁ : List Γ'),
    ∃ l₁' l₂' s',
      l₁.reverseAux (trPosNum m) = l₁'.reverseAux l₂' ∧
        Reaches₁ (TM2.step tr)
          { l := some (q₁.pred q₂), var := s, stk := elim (trPosNum m.succ ++ Γ'.cons :: trList v) l₁ c d }
          { l := some (unrev q₂), var := s', stk := elim (l₂' ++ Γ'.cons :: trList v) l₁' c d }
s : Option Γ'
l₁ : List Γ'
⊢ ∃ l₁' l₂' s',
    l₁.reverseAux (trPosNum m.bit1) = l₁'.reverseAux l₂' ∧
      Reaches₁ (TM2.step tr)
        { l := some (q₁.pred q₂), var := s, stk := elim (trPosNum m.bit1.succ ++ Γ'.cons :: trList v) l₁ c d }
        { l := some (unrev q₂), var := s', stk := elim (l₂' ++ Γ'.cons :: trList v) l₁' c d }

case cons.succ.pos.bit0
q₁ q₂ : Λ'
c d : List Γ'
v : List ℕ
n : ℕ
m : PosNum
IH :
  ∀ (s : Option Γ') (l₁ : List Γ'),
    ∃ l₁' l₂' s',
      l₁.reverseAux (trPosNum m) = l₁'.reverseAux l₂' ∧
        Reaches₁ (TM2.step tr)
          { l := some (q₁.pred q₂), var := s, stk := elim (trPosNum m.succ ++ Γ'.cons :: trList v) l₁ c d }
          { l := some (unrev q₂), var := s', stk := elim (l₂' ++ Γ'.cons :: trList v) l₁' c d }
s : Option Γ'
l₁ : List Γ'
⊢ ∃ l₁' l₂' s',
    l₁.reverseAux (trPosNum m.bit0) = l₁'.reverseAux l₂' ∧
      Reaches₁ (TM2.step tr)
        { l := some (q₁.pred q₂), var := s, stk := elim (trPosNum m.bit0.succ ++ Γ'.cons :: trList v) l₁ c d }
        { l := some (unrev q₂), var := s', stk := elim (l₂' ++ Γ'.cons :: trList v) l₁' c d }

Proof state:
q₁ q₂ : Λ'
c d : List Γ'
v : List ℕ
n : ℕ
m : PosNum
IH :
  ∀ (s : Option Γ') (l₁ : List Γ'),
    ∃ l₁' l₂' s',
      l₁.reverseAux (trPosNum m) = l₁'.reverseAux l₂' ∧
        Reaches₁ (TM2.step tr)
          { l := some (q₁.pred q₂), var := s, stk := elim (trPosNum m.succ ++ Γ'.cons :: trList v) l₁ c d }
          { l := some (unrev q₂), var := s', stk := elim (l₂' ++ Γ'.cons :: trList v) l₁' c d }
s : Option Γ'
l₁ : List Γ'
⊢ ∃ a l, trPosNum m = a :: l ∧ natEnd a = false

Tactic:
cases m <;> refine ⟨_, _, rfl, rfl⟩

Result:
no goals

Proof state:
case cons.succ
q₁ q₂ : Λ'
s : Option Γ'
c d : List Γ'
v : List ℕ
n : ℕ
⊢ Reaches₁ (TM2.step tr)
    { l := some (q₁.pred q₂), var := s, stk := elim (trPosNum (↑n).succ' ++ Γ'.cons :: trList v) [] c d }
    { l := some q₂, var := none,
      stk :=
        elim
          ((match ↑n with
            | Num.zero => []
            | Num.pos n => trPosNum n) ++
            Γ'.cons :: trList v)
          [] c d }

Tactic:
rcases (n : Num) with - | a

Result:
case cons.succ.zero
q₁ q₂ : Λ'
s : Option Γ'
c d : List Γ'
v : List ℕ
n : ℕ
⊢ Reaches₁ (TM2.step tr)
    { l := some (q₁.pred q₂), var := s, stk := elim (trPosNum Num.zero.succ' ++ Γ'.cons :: trList v) [] c d }
    { l := some q₂, var := none,
      stk :=
        elim
          ((match Num.zero with
            | Num.zero => []
            | Num.pos n => trPosNum n) ++
            Γ'.cons :: trList v)
          [] c d }

case cons.succ.pos
q₁ q₂ : Λ'
s : Option Γ'
c d : List Γ'
v : List ℕ
n : ℕ
a : PosNum
⊢ Reaches₁ (TM2.step tr)
    { l := some (q₁.pred q₂), var := s, stk := elim (trPosNum (Num.pos a).succ' ++ Γ'.cons :: trList v) [] c d }
    { l := some q₂, var := none,
      stk :=
        elim
          ((match Num.pos a with
            | Num.zero => []
            | Num.pos n => trPosNum n) ++
            Γ'.cons :: trList v)
          [] c d }

Proof state:
case nil
q₁ q₂ : Λ'
s : Option Γ'
c d : List Γ'
⊢ ∃ s',
    Reaches₁ (TM2.step tr) { l := some (q₁.pred q₂), var := s, stk := elim (trList []) [] c d }
      (Nat.rec { l := some q₁, var := s', stk := elim (trList [].tail) [] c d }
        (fun n x => { l := some q₂, var := s', stk := elim (trList (n :: [].tail)) [] c d }) [].headI)

Tactic:
refine ⟨none, TransGen.single ?_⟩

Result:
case nil
q₁ q₂ : Λ'
s : Option Γ'
c d : List Γ'
⊢ Nat.rec { l := some q₁, var := none, stk := elim (trList [].tail) [] c d }
      (fun n x => { l := some q₂, var := none, stk := elim (trList (n :: [].tail)) [] c d }) [].headI ∈
    TM2.step tr { l := some (q₁.pred q₂), var := s, stk := elim (trList []) [] c d }