theorem evaln_complete {c n x} : x ∈ eval c n ↔ ∃ k, x ∈ evaln k c n :=
  ⟨fun h => by
    rsuffices ⟨k, h⟩ : ∃ k, x ∈ evaln (k + 1) c n
    · exact ⟨k + 1, h⟩
    induction c generalizing n x <;> simp [eval, evaln, pure, PFun.pure, Seq.seq, Bind.bind] at h ⊢
    iterate 4 exact ⟨⟨_, le_rfl⟩, h.symm⟩
    case pair cf cg hf hg =>
      rcases h with ⟨x, hx, y, hy, rfl⟩
      rcases hf hx with ⟨k₁, hk₁⟩; rcases hg hy with ⟨k₂, hk₂⟩
      refine' ⟨max k₁ k₂, _⟩
      refine'
        ⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
          evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁, _,
          evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂, rfl⟩
    case comp cf cg hf hg =>
      rcases h with ⟨y, hy, hx⟩
      rcases hg hy with ⟨k₁, hk₁⟩; rcases hf hx with ⟨k₂, hk₂⟩
      refine' ⟨max k₁ k₂, _⟩
      exact
        ⟨le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁, _,
          evaln_mono (Nat.succ_le_succ <| le_max_left _ _) hk₁,
          evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂⟩
    case prec cf cg hf hg =>
      revert h
      generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
      induction' n₂ with m IH generalizing x n <;> simp
      · intro h
        rcases hf h with ⟨k, hk⟩
        exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
      · intro y hy hx
        rcases IH hy with ⟨k₁, nk₁, hk₁⟩
        rcases hg hx with ⟨k₂, hk₂⟩
        refine'
          ⟨(max k₁ k₂).succ,
            Nat.le_succ_of_le <| le_max_of_le_left <|
              le_trans (le_max_left _ (Nat.pair n₁ m)) nk₁, y,
            evaln_mono (Nat.succ_le_succ <| le_max_left _ _) _,
            evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_right _ _) hk₂⟩
        simp only [evaln._eq_8, bind, unpaired, unpair_pair, Option.mem_def, Option.bind_eq_some,
          Option.guard_eq_some', exists_and_left, exists_const]
        

Proof state:
case succ.intro.intro.intro
cf cg : Code
hf : ∀ {n x : ℕ}, x ∈ eval cf n → ∃ k, x ∈ evaln (k + 1) cf n
hg : ∀ {n x : ℕ}, x ∈ eval cg n → ∃ k, x ∈ evaln (k + 1) cg n
n₁ m : ℕ
IH :
  ∀ {n x : ℕ},
    x ∈ Nat.rec (eval cf n₁) (fun y IH => Part.bind IH fun i => eval cg (Nat.pair n₁ (Nat.pair y i))) m →
      ∃ k,
        n ≤ k ∧
          Nat.rec (evaln (k + 1) cf n₁)
              (fun n n_ih =>
                Option.bind (evaln k (prec cf cg) (Nat.pair n₁ n)) fun i =>
                  evaln (k + 1) cg (Nat.pair n₁ (Nat.pair n i)))
              m =
            some x
n x y : ℕ
hy : y ∈ Nat.rec (eval cf n₁) (fun y IH => Part.bind IH fun i => eval cg (Nat.pair n₁ (Nat.pair y i))) m
hx : x ∈ eval cg (Nat.pair n₁ (Nat.pair m y))
k₁ : ℕ
nk₁ : max n (Nat.pair n₁ m) ≤ k₁
hk₁ :
  Nat.rec (evaln (k₁ + 1) cf n₁)
      (fun n n_ih =>
        Option.bind (evaln k₁ (prec cf cg) (Nat.pair n₁ n)) fun i => evaln (k₁ + 1) cg (Nat.pair n₁ (Nat.pair n i)))
      m =
    some y
k₂ : ℕ
hk₂ : x ∈ evaln (k₂ + 1) cg (Nat.pair n₁ (Nat.pair m y))
⊢ Nat.pair n₁ m ≤ k₁ ∧
    Nat.rec (evaln (k₁ + 1) cf n₁)
        (fun n n_ih =>
          Option.bind (evaln k₁ (prec cf cg) (Nat.pair n₁ n)) fun i => evaln (k₁ + 1) cg (Nat.pair n₁ (Nat.pair n i)))
        m =
      some y

Next tactic:
exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩