-- Mathlib/Computability/PartrecCode.lean
theorem evaln_prim : Primrec fun a : (ℕ × Code) × ℕ => evaln a.1.1 a.1.2 a.2 :=
  have :
    Primrec₂ fun (_ : Unit) (n : ℕ) =>
      let a := ofNat (ℕ × Code) n
      (List.range a.1).map (evaln a.1 a.2) :=
    Primrec.nat_strong_rec _ (hG.comp Primrec.snd).to₂ fun _ p => by
      simp only [G, prod_ofNat_val, ofNat_nat, List.length_map, List.length_range,
        Nat.pair_unpair, Option.some_inj]
      refine List.map_congr_left fun n => ?_
      have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2)))

Proof state:
case succ.rfind'.some
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') →
      Nat.Partrec.Code.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
lf : encode cf < encode cf.rfind'
x : ℕ
⊢ Nat.rec (some (unpair n).2)
      (fun n_1 n_ih =>
        Nat.Partrec.Code.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 =
    if x = 0 then some (unpair n).2 else evaln k' cf.rfind' (Nat.pair (unpair n).1 ((unpair n).2 + 1))

Tactic:
cases x <;> simp [Nat.succ_ne_zero]

Result:
case succ.rfind'.some.succ
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') →
      Nat.Partrec.Code.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
lf : encode cf < encode cf.rfind'
n✝ : ℕ
⊢ Nat.Partrec.Code.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)) =
    evaln k' cf.rfind' (Nat.pair (unpair n).1 ((unpair n).2 + 1))

Proof state:
case succ.prec.intro.succ
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)) →
      Nat.Partrec.Code.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
lf : encode cf < encode (cf.prec cg)
lg : encode cg < encode (cf.prec cg)
n✝ : ℕ
⊢ ((Nat.Partrec.Code.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✝)).bind
      fun i =>
      Nat.Partrec.Code.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✝ i))) =
    (evaln k' (cf.prec cg) (Nat.pair (unpair n).1 n✝)).bind fun i =>
      evaln (k' + 1) cg (Nat.pair (unpair n).1 (Nat.pair n✝ i))

Tactic:
rw [hg (Nat.pair_lt_pair_left _ k'.lt_succ_self)]

Result:
case succ.prec.intro.succ
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)) →
      Nat.Partrec.Code.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
lf : encode cf < encode (cf.prec cg)
lg : encode cg < encode (cf.prec cg)
n✝ : ℕ
⊢ ((evaln k' (cf.prec cg) (Nat.pair (unpair n).1 n✝)).bind fun i =>
      Nat.Partrec.Code.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✝ i))) =
    (evaln k' (cf.prec cg) (Nat.pair (unpair n).1 n✝)).bind fun i =>
      evaln (k' + 1) cg (Nat.pair (unpair n).1 (Nat.pair n✝ i))

Proof state:
case succ.rfind'.some.succ
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') →
      Nat.Partrec.Code.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
lf : encode cf < encode cf.rfind'
n✝ : ℕ
⊢ Nat.Partrec.Code.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)) =
    evaln k' cf.rfind' (Nat.pair (unpair n).1 ((unpair n).2 + 1))

Tactic:
rw [hg (Nat.pair_lt_pair_left _ k'.lt_succ_self)]

Result:
no goals