-- Lean 4 proof step from Mathlib.Computability.TuringMachine.ToPartrec (goal)
Goal:
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'} (e : splitAtPred p (S k₁) = (L₁, o, L₂)),
    Reaches₁ (TM2.step (Γ := fun (x : K') ↦ Γ') tr) (TM2.Cfg.mk (Γ := fun (x : K') ↦ Γ') (some (Λ'.move p k₁ k₂ q)) s S)
      (TM2.Cfg.mk (Γ := fun (x : K') ↦ Γ') (some q) o
        (update (β := fun (k : K') ↦ List Γ') (update (β := fun (a : K') ↦ List Γ') 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
fst : List Γ'
fst_1 : Option Γ'
snd : List Γ'
e : (a' :: (fst, fst_1, snd).1, (fst, fst_1, snd).2.1, (fst, fst_1, snd).2.2) = (a :: L₁, o, L₂)
e₃ : splitAtPred p Sk = (fst, fst_1, snd)
⊢ TransGen (fun (a b : TM2.Cfg (fun (x : K') ↦ Γ') Λ' (Option Γ')) ↦ b ∈ TM2.step (Γ := fun (x : K') ↦ Γ') tr a)
    (bif
        match (motive := Option Γ' → Bool → (Γ' → Bool) → Bool) (a' :: Sk).head?, true, p with
        | some x, x_1, f => f x
        | none, y, x => y then
      TM2.Cfg.mk (Γ := fun (x : K') ↦ Γ') (some q) (a' :: Sk).head?
        (update (β := fun (k : K') ↦ List Γ') S k₁ (a' :: Sk).tail)
    else
      TM2.Cfg.mk (Γ := fun (x : K') ↦ Γ') (some (Λ'.move p k₁ k₂ q)) (a' :: Sk).head?
        (update (β := fun (k : K') ↦ List Γ') (update (β := fun (k : K') ↦ List Γ') S k₁ (a' :: Sk).tail) k₂
          ((a' :: Sk).head?.getD default :: update (β := fun (k : K') ↦ List Γ') S k₁ (a' :: Sk).tail k₂)))
    (TM2.Cfg.mk (Γ := fun (x : K') ↦ Γ') (some q) o
      (update (β := fun (k : K') ↦ List Γ') (update (β := fun (a : K') ↦ List Γ') S k₁ L₂) k₂
        ((a :: L₁).reverseAux (S k₂))))
have cons : ∀ (e₁ : S k₁ = a :: Sk) (e₂ : p a = false)
  (IH :
    ∀ {s : Option Γ'} {S : K' → List Γ'}
      (e : splitAtPred p (S k₁) = ((fst, fst_1, snd).1, (fst, fst_1, snd).2.1, (fst, fst_1, snd).2.2)),
      Reaches₁ (TM2.step (Γ := fun (x : K') ↦ Γ') tr)
        (TM2.Cfg.mk (Γ := fun (x : K') ↦ Γ') (some (Λ'.move p k₁ k₂ q)) s S)
        (TM2.Cfg.mk (Γ := fun (x : K') ↦ Γ') (some q) (fst, fst_1, snd).2.1
          (update (β := fun (k : K') ↦ List Γ') (update (β := fun (a : K') ↦ List Γ') S k₁ (fst, fst_1, snd).2.2) k₂
            ((fst, fst_1, snd).1.reverseAux (S k₂))))),
  TransGen (fun (a b : TM2.Cfg (fun (x : K') ↦ Γ') Λ' (Option Γ')) ↦ b ∈ TM2.step (Γ := fun (x : K') ↦ Γ') tr a)
    (bif
        match (motive := Option Γ' → Bool → (Γ' → Bool) → Bool) (a :: Sk).head?, true, p with
        | some x, x_1, f => f x
        | none, y, x => y then
      TM2.Cfg.mk (Γ := fun (x : K') ↦ Γ') (some q) (a :: Sk).head?
        (update (β := fun (k : K') ↦ List Γ') S k₁ (a :: Sk).tail)
    else
      TM2.Cfg.mk (Γ := fun (x : K') ↦ Γ') (some (Λ'.move p k₁ k₂ q)) (a :: Sk).head?
        (update (β := fun (k : K') ↦ List Γ') (update (β := fun (k : K') ↦ List Γ') S k₁ (a :: Sk).tail) k₂
          ((a :: Sk).head?.getD default :: update (β := fun (k : K') ↦ List Γ') S k₁ (a :: Sk).tail k₂)))
    (TM2.Cfg.mk (Γ := fun (x : K') ↦ Γ') (some q) (fst, fst_1, snd).2.1
      (update (β := fun (k : K') ↦ List Γ') (update (β := fun (a : K') ↦ List Γ') S k₁ (fst, fst_1, snd).2.2) k₂
        ((a :: (fst, fst_1, snd).1).reverseAux (S k₂))))
Tactic: cases e
Cleared hypotheses: o, L₂, L₁, IH, a', e₁, e₂, e