-- Lean 4 proof step from Mathlib.Computability.PartrecCode (goal)
Goal:
c : Code
n x : ℕ
h : x ∈ c.eval n
⊢ ∃ (k : ℕ), x ∈ evaln (k + 1) c n
have pair : ∀ (cf cg : Code) (hf : ∀ {n x : ℕ} (h : x ∈ cf.eval n), ∃ (k : ℕ), x ∈ evaln (k + 1) cf n)
  (hg : ∀ {n x : ℕ} (h : x ∈ cg.eval n), ∃ (k : ℕ), x ∈ evaln (k + 1) cg n) {n x : ℕ} (h : x ∈ (cf.pair cg).eval n),
  ∃ (k : ℕ), x ∈ evaln (k + 1) (cf.pair cg) n
Tactic: induction c generalizing n x with
      simp [eval, evaln, pure, PFun.pure, Seq.seq, Option.bind_eq_some_iff] at h ⊢
  | 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⟩
  | 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⟩
  | prec cf cg hf hg =>
    revert h
    generalize n.unpair.1 = n₁; generalize n.unpair.2 = n₂
    induction n₂ generalizing x n with simp [Option.bind_eq_some_iff]
    | zero =>
      intro h
      rcases hf h with ⟨k, hk⟩
      exact ⟨_, le_max_left _ _, evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk⟩
    | succ m IH =>
      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_iff,
        Option.guard_eq_some', exists_and_left, exists_const]
      exact ⟨le_trans (le_max_right _ _) nk₁, hk₁⟩
  | rfind' cf hf =>
    rcases h with ⟨y, ⟨hy₁, hy₂⟩, rfl⟩
    suffices ∃ k, y + n.unpair.2 ∈ evaln (k + 1) (rfind' cf) (Nat.pair n.unpair.1 n.unpair.2) by
      simpa [evaln, Option.bind_eq_some_iff]
    revert hy₁ hy₂
    generalize n.unpair.2 = m
    intro hy₁ hy₂
    induction y generalizing m with simp [evaln, Option.bind_eq_some_iff]
    | zero =>
      simp at hy₁
      rcases hf hy₁ with ⟨k, hk⟩
      exact ⟨_, Nat.le_of_lt_succ <| evaln_bound hk, _, hk, by simp⟩
    | succ y IH =>
      rcases hy₂ (Nat.succ_pos _) with ⟨a, ha, a0⟩
      rcases hf ha with ⟨k₁, hk₁⟩
      rcases IH m.succ (by simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using hy₁)
          fun {i} hi => by
          simpa [Nat.succ_eq_add_one, add_comm, add_left_comm] using
            hy₂ (Nat.succ_lt_succ hi) with
        ⟨k₂, hk₂⟩
      use (max k₁ k₂).succ
      rw [zero_add] at hk₁
      use Nat.le_succ_of_le <| le_max_of_le_left <| Nat.le_of_lt_succ <| evaln_bound hk₁
      use a
      use evaln_mono (Nat.succ_le_succ <| Nat.le_succ_of_le <| le_max_left _ _) hk₁
      simpa [a0, add_comm, add_left_comm] using
        evaln_mono (Nat.succ_le_succ <| le_max_right _ _) hk₂
  | _ => exact ⟨⟨_, le_rfl⟩, h.symm⟩
Cleared hypotheses: c, n, x, h