/-- The map `f ↦ (x ↦ B (f x) (g x))` as a continuous `𝕜`-linear map on Schwartz space,
where `B` is a continuous `𝕜`-linear map and `g` is a function of temperate growth. -/
def bilinLeftCLM (B : E →L[ℝ] F →L[ℝ] G) {g : D → F} (hg : g.HasTemperateGrowth) :
    𝓢(D, E) →L[ℝ] 𝓢(D, G) :=
  -- Todo (after port): generalize to `B : E →L[𝕜] F →L[𝕜] G` and `𝕜`-linear
    mkCLM
    (fun f x => B (f x) (g x))
    (fun _ _ _ => by
      simp only [map_add, add_left_inj, Pi.add_apply, eq_self_iff_true,
        ContinuousLinearMap.add_apply])
    (fun _ _ _ => by
      simp only [smul_apply, map_smul, ContinuousLinearMap.coe_smul', Pi.smul_apply,
        RingHom.id_apply])
    (fun f => (B.isBoundedBilinearMap.contDiff.restrict_scalars ℝ).comp (f.smooth'.prod hg.1))
    (by
      -- Porting note: rewrite this proof with `rel_congr`
      rintro ⟨k, n⟩
      rcases hg.norm_iteratedFDeriv_le_uniform_aux n with ⟨l, C, hC, hgrowth⟩
      use
        Finset.Iic (l + k, n), ‖B‖ * ((n : ℝ) + (1 : ℝ)) * n.choose (n / 2) * (C * 2 ^ (l + k)),
        by positivity
      intro f x
      have hxk : 0 ≤ ‖x‖ ^ k := by positivity
      have hnorm_mul :=
        ContinuousLinearMap.norm_iteratedFDeriv_le_of_bilinear B f.smooth' hg.1 x (n := n) le_top
      refine' le_trans (mul_le_mul_of_nonneg_left hnorm_mul hxk) _
      rw [← mul_assoc (‖x‖ ^ k), mul_comm (‖x‖ ^ k)]
      simp_rw [mul_assoc ‖B‖]
      refine' mul_le_mul_of_nonneg_left _ (by positivity)
      rw [Finset.mul_sum]
      have : (∑ _x : ℕ in Finset.range (n + 1), (1 : ℝ)) = n + 1 := by simp
      repeat rw [mul_assoc ((n : ℝ) + 1)]
      rw [← this, Finset.sum_mul]
      refine' Finset.sum_le_sum fun i hi => _
      simp only [one_mul]
      rw [← mul_assoc, mul_comm (‖x‖ ^ k), mul_assoc, mul_assoc, mul_assoc]
      refine' mul_le_mul _ _ (by positivity) (by positivity)
      · norm_cast
        exact i.choose_le_middle n
      specialize hgrowth (n - i) (by simp only [tsub_le_self]) x
      rw [← mul_assoc]
      refine' le_trans (mul_le_mul_of_nonneg_left hgrowth (by positivity)) _
      rw [mul_comm _ (C * _), mul_assoc, mul_assoc C]
      refine' mul_le_mul_of_nonneg_left _ hC
      rw [mul_comm _ (‖x‖ ^ k)]
      rw [← mul_assoc]
      rw [Finset.mem_range_succ_iff] at hi
      change i ≤ (l + k, n).snd at hi
      refine' le_trans _ (one_add_le_sup_seminorm_apply le_rfl hi f x)
      refine' mul_le_mul_of_nonneg_right _ (norm_nonneg _)
      

Proof state:
case right.refine'_2
𝕜 : Type u_1
𝕜' : Type u_2
D : Type u_3
E : Type u_4
F : Type u_5
G : Type u_6
inst✝⁷ : NormedAddCommGroup E
inst✝⁶ : NormedSpace ℝ E
inst✝⁵ : NormedAddCommGroup F
inst✝⁴ : NormedSpace ℝ F
inst✝³ : NormedAddCommGroup D
inst✝² : NormedSpace ℝ D
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
B : E →L[ℝ] F →L[ℝ] G
g : D → F
hg : Function.HasTemperateGrowth g
k n l : ℕ
C : ℝ
hC : 0 ≤ C
f : 𝓢(D, E)
x : D
hxk : 0 ≤ ‖x‖ ^ k
hnorm_mul :
  ‖iteratedFDeriv ℝ n (fun y => (B (toFun f y)) (g y)) x‖ ≤
    ‖B‖ *
      ∑ i in Finset.range (n + 1), ↑(Nat.choose n i) * ‖iteratedFDeriv ℝ i f.toFun x‖ * ‖iteratedFDeriv ℝ (n - i) g x‖
this : ∑ _x in Finset.range (n + 1), 1 = ↑n + 1
i : ℕ
hgrowth : ‖iteratedFDeriv ℝ (n - i) g x‖ ≤ C * (1 + ‖x‖) ^ l
hi : i ≤ (l + k, n).2
⊢ (1 + ‖x‖) ^ l * ‖x‖ ^ k ≤ (1 + ‖x‖) ^ (l + k, n).1

Next tactic:
rw [pow_add]