Module: Mathlib.Computability.PartrecCode

Goal state before:
pp: case succ.prec
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf cg : Code
hg :
  ∀ {k' : ℕ} {c' : Code} {n : ℕ},
    Nat.pair k' (encode c') < Nat.pair k (encode (cf.prec cg)) →
      lup
          (List.map (fun n ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
            (List.range (Nat.pair k (encode (cf.prec cg)))))
          (k', c') n =
        evaln k' c' n
⊢ Nat.rec
      (lup
        (List.map (fun n ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
          (List.range (Nat.pair (k' + 1) (encode (cf.prec cg)))))
        (k' + 1, cf) (unpair n).1)
      (fun n_1 n_ih ↦
        (lup
              (List.map (fun n ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                (List.range (Nat.pair (k' + 1) (encode (cf.prec cg)))))
              (k', cf.prec cg) (Nat.pair (unpair n).1 n_1)).bind
          fun i ↦
          lup
            (List.map (fun n ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
              (List.range (Nat.pair (k' + 1) (encode (cf.prec cg)))))
            (k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
      (unpair n).2 =
    Nat.rec (evaln (k' + 1) cf (unpair n).1)
      (fun n_1 n_ih ↦
        (evaln k' (cf.prec cg) (Nat.pair (unpair n).1 n_1)).bind fun i ↦
          evaln (k' + 1) cg (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
      (unpair n).2

pp: case succ.rfind'
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf : Code
hg :
  ∀ {k' : ℕ} {c' : Code} {n : ℕ},
    Nat.pair k' (encode c') < Nat.pair k (encode cf.rfind') →
      lup
          (List.map (fun n ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
            (List.range (Nat.pair k (encode cf.rfind'))))
          (k', c') n =
        evaln k' c' n
⊢ ((lup
          (List.map (fun n ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
            (List.range (Nat.pair (k' + 1) (encode cf.rfind'))))
          (k' + 1, cf) n).bind
      fun x ↦
      Nat.rec (some (unpair n).2)
        (fun n_1 n_ih ↦
          lup
            (List.map (fun n ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
              (List.range (Nat.pair (k' + 1) (encode cf.rfind'))))
            (k', cf.rfind') (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
        x) =
    (evaln (k' + 1) cf n).bind fun a ↦
      if a = 0 then some (unpair n).2 else evaln k' cf.rfind' (Nat.pair (unpair n).1 ((unpair n).2 + 1))

Tactic:
· obtain ⟨lf, lg⟩ := encode_lt_prec cf cg
        rw [hg (Nat.pair_lt_pair_right _ lf)]
        cases n.unpair.2
        · rfl
        simp only
        rw [hg (Nat.pair_lt_pair_left _ k'.lt_succ_self)]
        cases evaln k' _ _
        · rfl
        simp [k, hg (Nat.pair_lt_pair_right _ lg)]

Goal state after:
case succ.rfind'
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k' : ℕ
k : ℕ := k' + 1
nk : n ≤ k'
cf : Code
hg :
  ∀ {k' : ℕ} {c' : Code} {n : ℕ},
    Nat.pair k' (encode c') < Nat.pair k (encode cf.rfind') →
      lup
          (List.map (fun n ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
            (List.range (Nat.pair k (encode cf.rfind'))))
          (k', c') n =
        evaln k' c' n
⊢ ((lup
          (List.map (fun n ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
            (List.range (Nat.pair (k' + 1) (encode cf.rfind'))))
          (k' + 1, cf) n).bind
      fun x ↦
      Nat.rec (some (unpair n).2)
        (fun n_1 n_ih ↦
          lup
            (List.map (fun n ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
              (List.range (Nat.pair (k' + 1) (encode cf.rfind'))))
            (k', cf.rfind') (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
        x) =
    (evaln (k' + 1) cf n).bind fun a ↦
      if a = 0 then some (unpair n).2 else evaln k' cf.rfind' (Nat.pair (unpair n).1 ((unpair n).2 + 1))