-- Lean 4 proof step from Mathlib.MeasureTheory.Integral.MeanInequalities (goal)
Goal:
α : Type u_2
ι : Type u_3
inst : MeasurableSpace α
μ : Measure α
s : Finset ι
f : ι → α → ℝ≥0∞
hf : ∀ i ∈ s, AEMeasurable (f i) μ
p : ι → ℝ
hp : Eq (α := ℝ) (∑ i ∈ s, p i) 1
h2p : ∀ i ∈ s, 0 ≤ p i
empty :
  ∀ (hf : ∀ (i : ι), Membership.mem (γ := Finset ι) ∅ i → AEMeasurable (f i) μ) {p : ι → ℝ}
    (hp : Eq (α := ℝ) (∑ i ∈ ∅, p i) 1) (h2p : ∀ (i : ι), Membership.mem (γ := Finset ι) ∅ i → 0 ≤ p i),
    ∫⁻ (a : α), ∏ i ∈ ∅, f i a ^ p i ∂μ ≤ ∏ i ∈ ∅, (∫⁻ (a : α), f i a ∂μ) ^ p i
⊢ ∫⁻ (a : α), ∏ i ∈ s, f i a ^ p i ∂μ ≤ ∏ i ∈ s, (∫⁻ (a : α), f i a ∂μ) ^ p i
have insert : ∀ (i₀ : ι) (s : Finset ι) (hi₀ : i₀ ∉ s)
  (ih :
    ∀ (hf : ∀ i ∈ s, AEMeasurable (f i) μ) {p : ι → ℝ} (hp : Eq (α := ℝ) (∑ i ∈ s, p i) 1) (h2p : ∀ i ∈ s, 0 ≤ p i),
      ∫⁻ (a : α), ∏ i ∈ s, f i a ^ p i ∂μ ≤ ∏ i ∈ s, (∫⁻ (a : α), f i a ∂μ) ^ p i)
  (hf : ∀ i ∈ insert i₀ s, AEMeasurable (f i) μ) {p : ι → ℝ} (hp : Eq (α := ℝ) (∑ i ∈ insert i₀ s, p i) 1)
  (h2p : ∀ i ∈ insert i₀ s, 0 ≤ p i),
  ∫⁻ (a : α), ∏ i ∈ insert i₀ s, f i a ^ p i ∂μ ≤ ∏ i ∈ insert i₀ s, (∫⁻ (a : α), f i a ∂μ) ^ p i
Tactic: classical
  induction s using Finset.induction generalizing p with
  | empty =>
    simp at hp
  | insert i₀ s hi₀ ih =>
    rcases eq_or_ne (p i₀) 1 with h2i₀ | h2i₀
    · simp only [hi₀, not_false_eq_true, prod_insert]
      have h2p : ∀ i ∈ s, p i = 0 := by
        simpa [hi₀, h2i₀, sum_eq_zero_iff_of_nonneg (fun i hi ↦ h2p i <| mem_insert_of_mem hi)]
          using hp
      calc ∫⁻ a, f i₀ a ^ p i₀ * ∏ i ∈ s, f i a ^ p i ∂μ
          = ∫⁻ a, f i₀ a ^ p i₀ * ∏ i ∈ s, 1 ∂μ := by
            congr! 3 with x
            apply prod_congr rfl fun i hi ↦ by rw [h2p i hi, ENNReal.rpow_zero]
        _ ≤ (∫⁻ a, f i₀ a ∂μ) ^ p i₀ * ∏ i ∈ s, 1 := by simp [h2i₀]
        _ = (∫⁻ a, f i₀ a ∂μ) ^ p i₀ * ∏ i ∈ s, (∫⁻ a, f i a ∂μ) ^ p i := by
            congr 1
            apply prod_congr rfl fun i hi ↦ by rw [h2p i hi, ENNReal.rpow_zero]
    · have hpi₀ : 0 ≤ 1 - p i₀ := by
        simp_rw [sub_nonneg, ← hp, single_le_sum h2p (mem_insert_self ..)]
      have h2pi₀ : 1 - p i₀ ≠ 0 := by
        rwa [sub_ne_zero, ne_comm]
      let q := fun i ↦ p i / (1 - p i₀)
      have hq : ∑ i ∈ s, q i = 1 := by
        rw [← Finset.sum_div, ← sum_insert_sub hi₀, hp, div_self h2pi₀]
      have h2q : ∀ i ∈ s, 0 ≤ q i :=
        fun i hi ↦ div_nonneg (h2p i <| mem_insert_of_mem hi) hpi₀
      calc ∫⁻ a, ∏ i ∈ insert i₀ s, f i a ^ p i ∂μ
          = ∫⁻ a, f i₀ a ^ p i₀ * ∏ i ∈ s, f i a ^ p i ∂μ := by simp [hi₀]
        _ = ∫⁻ a, f i₀ a ^ p i₀ * (∏ i ∈ s, f i a ^ q i) ^ (1 - p i₀) ∂μ := by
            simp [q, ← ENNReal.prod_rpow_of_nonneg hpi₀, ← ENNReal.rpow_mul,
              div_mul_cancel₀ (h := h2pi₀)]
        _ ≤ (∫⁻ a, f i₀ a ∂μ) ^ p i₀ * (∫⁻ a, ∏ i ∈ s, f i a ^ q i ∂μ) ^ (1 - p i₀) := by
            apply ENNReal.lintegral_mul_norm_pow_le
            · exact hf i₀ <| mem_insert_self ..
            · exact s.aemeasurable_fun_prod fun i hi ↦ (hf i <| mem_insert_of_mem hi).pow_const _
            · exact h2p i₀ <| mem_insert_self ..
            · exact hpi₀
            · apply add_sub_cancel
        _ ≤ (∫⁻ a, f i₀ a ∂μ) ^ p i₀ * (∏ i ∈ s, (∫⁻ a, f i a ∂μ) ^ q i) ^ (1 - p i₀) := by
            gcongr -- behavior of gcongr is heartbeat-dependent, which makes code really fragile...
            exact ih (fun i hi ↦ hf i <| mem_insert_of_mem hi) hq h2q
        _ = (∫⁻ a, f i₀ a ∂μ) ^ p i₀ * ∏ i ∈ s, (∫⁻ a, f i a ∂μ) ^ p i := by
            simp [q, ← ENNReal.prod_rpow_of_nonneg hpi₀, ← ENNReal.rpow_mul,
              div_mul_cancel₀ (h := h2pi₀)]
        _ = ∏ i ∈ insert i₀ s, (∫⁻ a, f i a ∂μ) ^ p i := by simp [hi₀]
Cleared hypotheses: s, hf, p, hp, h2p