/-- 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 

Proof state:
case succ
x✝ : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
c : Code
k' : ℕ
nk : n ∈ List.range (Nat.succ k')
k : ℕ := k' + 1
⊢ rec (some 0) (some (Nat.succ n)) (some (unpair n).1) (some (unpair n).2)
      (fun cf cg x x => do
        let x ←
          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 c))))
              (k' + 1, cf) n
        let y ←
          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 c))))
              (k' + 1, cg) n
        some (Nat.pair x y))
      (fun cf cg x x => do
        let x ←
          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 c))))
              (k' + 1, cg) 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 c))))
            (k' + 1, cf) x)
      (fun cf cg x x =>
        Nat.rec
          (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 c))))
            (k' + 1, cf) (unpair n).1)
          (fun n_1 n_ih => do
            let 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 c))))
                  (k', c) (Nat.pair (unpair n).1 n_1)
            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 c))))
                (k' + 1, cg) (Nat.pair (unpair n).1 (Nat.pair n_1 i)))
          (unpair n).2)
      (fun cf x => do
        let x ←
          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 c))))
              (k' + 1, cf) 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 c))))
                (k', c) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
            x)
      c =
    evaln (k' + 1) c n

Next tactic:
simp only [List.mem_range, lt_succ_iff] at nk