/-- The `Nat.Partrec.Code.evaln` function is primitive recursive. -/
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 fun n => ?_
      have : List.range p = List.range (Nat.pair p.unpair.1 (encode (ofNat Code p.unpair.2))) := by
        simp
      rw [this]
      generalize p.unpair.1 = k
      generalize ofNat Code p.unpair.2 = c
      intro nk
      cases' k with k'
      · simp [evaln]
      let k := k' + 1
      simp only [show k'.succ = k from rfl]
      simp? [Nat.lt_succ_iff] at nk says simp only [List.mem_range, lt_succ_iff] at nk
      have hg :
        ∀ {k' c' n},
          Nat.pair k' (encode c') < Nat.pair k (encode c) →
            lup ((List.range (Nat.pair k (encode c))).map fun n =>
              (List.range n.unpair.1).map (evaln n.unpair.1 (ofNat Code n.unpair.2))) (k', c') n =
            evaln k' c' n := by
        intro k₁ c₁ n₁ hl
        simp [lup, List.get?_range hl, evaln_map, Bind.bind]
      cases' c with cf cg cf cg cf cg cf <;>
        simp [evaln, nk, Bind.bind, Functor.map, Seq.seq, pure]
      · cases' encode_lt_pair cf cg with lf lg
        rw [hg (Nat.pair_lt_pair_right _ lf), hg (Nat.pair_lt_pair_right _ lg)]
        cases evaln k cf n
        · rfl
        cases evaln k cg n <;> rfl
      · cases' encode_lt_comp cf cg with lf lg
        rw [hg (Nat.pair_lt_pair_right _ lg)]
        cases evaln k cg n
        · rfl
        simp [hg (Nat.pair_lt_pair_right _ lf)]
      · cases' encode_lt_prec cf cg with lf lg
        rw [hg (Nat.pair_lt_pair_right _ lf)]
        cases n.unpair.2
        · rfl
        simp only [decode_eq_ofNat, Option.some.injEq]
        rw [hg (Nat.pair_lt_pair_left _ k'.lt_succ_self)]
        cases evaln k' _ _
        · rfl
        simp [hg (Nat.pair_lt_pair_right _ lg)]
      · have lf := encode_lt_rfind' cf
        rw [hg (Nat.pair_lt_pair_right _ lf)]
        

Proof state:
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 (rfind' cf)) →
      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 (rfind' cf)))))
          (k', c') n =
        evaln k' c' n
lf : encode cf < encode (rfind' cf)
⊢ (Option.bind (evaln k cf n) fun x =>
      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 (rfind' cf)))))
            (k', rfind' cf) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
        x) =
    Option.bind (evaln (k' + 1) cf n) fun x =>
      if x = 0 then some (unpair n).2 else evaln k' (rfind' cf) (Nat.pair (unpair n).1 ((unpair n).2 + 1))

Next tactic:
cases' evaln k cf n with x