private theorem hG : Primrec G := by
  have a := (Primrec.ofNat (ℕ × Code)).comp (Primrec.list_length (α := List (Option ℕ)))
  have k := Primrec.fst.comp a
  refine' Primrec.option_some.comp (Primrec.list_map (Primrec.list_range.comp k) (_ : Primrec _))
  replace k := k.comp (Primrec.fst (β := ℕ))
  have n := Primrec.snd (α := List (List (Option ℕ))) (β := ℕ)
  refine' Primrec.nat_casesOn k (_root_.Primrec.const Option.none) (_ : Primrec _)
  have k := k.comp (Primrec.fst (β := ℕ))
  have n := n.comp (Primrec.fst (β := ℕ))
  have k' := Primrec.snd (α := List (List (Option ℕ)) × ℕ) (β := ℕ)
  have c := Primrec.snd.comp (a.comp <| (Primrec.fst (β := ℕ)).comp (Primrec.fst (β := ℕ)))
  apply
    Nat.Partrec.Code.rec_prim c
      (_root_.Primrec.const (some 0))
      (Primrec.option_some.comp (_root_.Primrec.succ.comp n))
      (Primrec.option_some.comp (Primrec.fst.comp <| Primrec.unpair.comp n))
      (Primrec.option_some.comp (Primrec.snd.comp <| Primrec.unpair.comp n))
  · have L := (Primrec.fst.comp Primrec.fst).comp
      (Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
        (β := Code × Code × Option ℕ × Option ℕ))
    have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
    have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
    have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
        (β := Code × Code × Option ℕ × Option ℕ))
    have cg := (Primrec.fst.comp Primrec.snd).comp
      (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
        (β := Code × Code × Option ℕ × Option ℕ))
    refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cf).pair n) ?_
    unfold Primrec₂
    conv =>
      congr
      · ext p
        dsimp only []
        erw [Option.bind_eq_bind, ← Option.map_eq_bind]
    refine Primrec.option_map ((hlup.comp <| L.pair <| (k.pair cg).pair n).comp Primrec.fst) ?_
    unfold Primrec₂
    exact Primrec₂.natPair.comp (Primrec.snd.comp Primrec.fst) Primrec.snd
  · have L := (Primrec.fst.comp Primrec.fst).comp
      (Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
        (β := Code × Code × Option ℕ × Option ℕ))
    have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
    have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
    have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
        (β := Code × Code × Option ℕ × Option ℕ))
    have cg := (Primrec.fst.comp Primrec.snd).comp
      (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
        (β := Code × Code × Option ℕ × Option ℕ))
    refine Primrec.option_bind (hlup.comp <| L.pair <| (k.pair cg).pair n) ?_
    unfold Primrec₂
    have h :=
      hlup.comp ((L.comp Primrec.fst).pair <| ((k.pair cf).comp Primrec.fst).pair Primrec.snd)
    exact h
  · have L := (Primrec.fst.comp Primrec.fst).comp
      (Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
        (β := Code × Code × Option ℕ × Option ℕ))
    have k := k.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
    have n := n.comp (Primrec.fst (β := Code × Code × Option ℕ × Option ℕ))
    have cf := Primrec.fst.comp (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
        (β := Code × Code × Option ℕ × Option ℕ))
    have cg := (Primrec.fst.comp Primrec.snd).comp
      (Primrec.snd (α := (List (List (Option ℕ)) × ℕ) × ℕ)
        (β := Code × Code × Option ℕ × Option ℕ))
    have z := Primrec.fst.comp (Primrec.unpair.comp n)
    refine'
      Primrec.nat_casesOn (Primrec.snd.comp (Primrec.unpair.comp n))
        (hlup.comp <| L.pair <| (k.pair cf).pair z)
        (_ : Primrec _)
    have L := L.comp (Primrec.fst (β := ℕ))
    have z := z.comp (Primrec.fst (β := ℕ))
    have y := Primrec.snd
      (α := ((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) (β := ℕ)
    have h₁ := hlup.comp <| L.pair <| (((k'.pair c).comp Primrec.fst).comp Primrec.fst).pair
      (Primrec₂.natPair.comp z y)
    refine' Primrec.option_bind h₁ (_ : Primrec _)
    have z := z.comp (Primrec.fst (β := ℕ))
    have y := y.comp (Primrec.fst (β := ℕ))
    have i := Primrec.snd
      (α := (((List (List (Option ℕ)) × ℕ) × ℕ) × Code × Code × Option ℕ × Option ℕ) × ℕ)
      (β := ℕ)
    have h₂ := hlup.comp ((L.comp Primrec.fst).pair <|
      ((k.pair cg).comp <| Primrec.fst.comp Primrec.fst).pair <|
        Primrec₂.natPair.comp z <| Primrec₂.natPair.comp y i)
    exact h₂
  · 

Proof state:
case hrf
a : Primrec fun a => ofNat (ℕ × Code) (List.length a)
k✝ : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1)).1
n✝ : Primrec Prod.snd
k : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).1
n : Primrec fun a => a.1.2
k' : Primrec Prod.snd
c : Primrec fun a => (ofNat (ℕ × Code) (List.length a.1.1)).2
⊢ Primrec fun a =>
    let z := (unpair a.1.1.2).1;
    let m := (unpair a.1.1.2).2;
    do
    let x ← Nat.Partrec.Code.lup a.1.1.1 ((ofNat (ℕ × Code) (List.length a.1.1.1)).1, a.2.1) (Nat.pair z m)
    Nat.casesOn x (some m) fun x =>
        Nat.Partrec.Code.lup a.1.1.1 (a.1.2, (ofNat (ℕ × Code) (List.length a.1.1.1)).2) (Nat.pair z (m + 1))

Next tactic:
have L := (Primrec.fst.comp Primrec.fst).comp
      (Primrec.fst (α := (List (List (Option ℕ)) × ℕ) × ℕ)
        (β := Code × Option ℕ))