-- Mathlib/Analysis/Calculus/ContDiff/Bounds.lean
theorem ContinuousLinearMap.norm_iteratedFDerivWithin_le_of_bilinear (B : E →L[𝕜] F →L[𝕜] G)
    {f : D → E} {g : D → F} {N : WithTop ℕ∞} {s : Set D} {x : D} (hf : ContDiffOn 𝕜 N f s)
    (hg : ContDiffOn 𝕜 N g s) (hs : UniqueDiffOn 𝕜 s) (hx : x ∈ s) {n : ℕ} (hn : n ≤ N) :
    ‖iteratedFDerivWithin 𝕜 n (fun y => B (f y) (g y)) s x‖ ≤
      ‖B‖ * ∑ i ∈ Finset.range (n + 1), (n.choose i : ℝ) * ‖iteratedFDerivWithin 𝕜 i f s x‖ *
        ‖iteratedFDerivWithin 𝕜 (n - i) g s x‖

Proof state:
𝕜 : Type u_1
inst✝⁸ : NontriviallyNormedField 𝕜
D : Type uD
inst✝⁷ : NormedAddCommGroup D
inst✝⁶ : NormedSpace 𝕜 D
E : Type uE
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
F : Type uF
inst✝³ : NormedAddCommGroup F
inst✝² : NormedSpace 𝕜 F
G : Type uG
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace 𝕜 G
B : E →L[𝕜] F →L[𝕜] G
f : D → E
g : D → F
N : WithTop ℕ∞
s : Set D
x : D
hf : ContDiffOn 𝕜 N f s
hg : ContDiffOn 𝕜 N g s
hs : UniqueDiffOn 𝕜 s
hx : x ∈ s
n : ℕ
hn : ↑n ≤ N
Du : Type (max uD uE uF uG) := ULift.{max uE uF uG, uD} D
Eu : Type (max uD uE uF uG) := ULift.{max uD uF uG, uE} E
Fu : Type (max uD uE uF uG) := ULift.{max uD uE uG, uF} F
Gu : Type (max uD uE uF uG) := ULift.{max uD uE uF, uG} G
isoD : Du ≃ₗᵢ[𝕜] D
isoE : Eu ≃ₗᵢ[𝕜] E
isoF : Fu ≃ₗᵢ[𝕜] F
isoG : Gu ≃ₗᵢ[𝕜] G
fu : Du → Eu := ⇑isoE.symm ∘ f ∘ ⇑isoD
gu : Du → Fu := ⇑isoF.symm ∘ g ∘ ⇑isoD
Bu₀ : Eu →L[𝕜] Fu →L[𝕜] G :=
  ((B.comp ↑{ toLinearEquiv := isoE.toLinearEquiv, continuous_toFun := ⋯, continuous_invFun := ⋯ }).flip.comp
      ↑{ toLinearEquiv := isoF.toLinearEquiv, continuous_toFun := ⋯, continuous_invFun := ⋯ }).flip
Bu : Eu →L[𝕜] Fu →L[𝕜] Gu :=
  ((compL 𝕜 Eu (Fu →L[𝕜] G) (Fu →L[𝕜] Gu))
      ((compL 𝕜 Fu G Gu) ↑{ toLinearEquiv := isoG.symm.toLinearEquiv, continuous_toFun := ⋯, continuous_invFun := ⋯ }))
    Bu₀
hBu :
  Bu =
    ((compL 𝕜 Eu (Fu →L[𝕜] G) (Fu →L[𝕜] Gu))
        ((compL 𝕜 Fu G Gu)
          ↑{ toLinearEquiv := isoG.symm.toLinearEquiv, continuous_toFun := ⋯, continuous_invFun := ⋯ }))
      Bu₀
Bu_eq : (fun y => (Bu (fu y)) (gu y)) = ⇑isoG.symm ∘ (fun y => (B (f y)) (g y)) ∘ ⇑isoD
Bu_le : ‖Bu‖ ≤ ‖B‖
su : Set Du := ⇑isoD ⁻¹' s
hsu : UniqueDiffOn 𝕜 su
xu : Du := isoD.symm x
hxu : xu ∈ su
xu_x : isoD xu = x
hfu : ContDiffOn 𝕜 (↑n) fu su
hgu : ContDiffOn 𝕜 (↑n) gu su
Nfu : ∀ (i : ℕ), ‖iteratedFDerivWithin 𝕜 i fu su xu‖ = ‖iteratedFDerivWithin 𝕜 i f s x‖
⊢ ‖iteratedFDerivWithin 𝕜 n (fun y => (B (f y)) (g y)) s x‖ ≤
    ‖B‖ *
      ∑ i ∈ Finset.range (n + 1),
        ↑(n.choose i) * ‖iteratedFDerivWithin 𝕜 i f s x‖ * ‖iteratedFDerivWithin 𝕜 (n - i) g s x‖

Tactic:
have Ngu : ∀ i, ‖iteratedFDerivWithin 𝕜 i gu su xu‖ = ‖iteratedFDerivWithin 𝕜 i g s x‖ := by
  intro i
  rw [LinearIsometryEquiv.norm_iteratedFDerivWithin_comp_left _ _ hsu hxu]
  rw [LinearIsometryEquiv.norm_iteratedFDerivWithin_comp_right _ _ hs, xu_x]
  rwa [← xu_x] at hx

Result:
𝕜 : Type u_1
inst✝⁸ : NontriviallyNormedField 𝕜
D : Type uD
inst✝⁷ : NormedAddCommGroup D
inst✝⁶ : NormedSpace 𝕜 D
E : Type uE
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
F : Type uF
inst✝³ : NormedAddCommGroup F
inst✝² : NormedSpace 𝕜 F
G : Type uG
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace 𝕜 G
B : E →L[𝕜] F →L[𝕜] G
f : D → E
g : D → F
N : WithTop ℕ∞
s : Set D
x : D
hf : ContDiffOn 𝕜 N f s
hg : ContDiffOn 𝕜 N g s
hs : UniqueDiffOn 𝕜 s
hx : x ∈ s
n : ℕ
hn : ↑n ≤ N
Du : Type (max uD uE uF uG) := ULift.{max uE uF uG, uD} D
Eu : Type (max uD uE uF uG) := ULift.{max uD uF uG, uE} E
Fu : Type (max uD uE uF uG) := ULift.{max uD uE uG, uF} F
Gu : Type (max uD uE uF uG) := ULift.{max uD uE uF, uG} G
isoD : Du ≃ₗᵢ[𝕜] D
isoE : Eu ≃ₗᵢ[𝕜] E
isoF : Fu ≃ₗᵢ[𝕜] F
isoG : Gu ≃ₗᵢ[𝕜] G
fu : Du → Eu := ⇑isoE.symm ∘ f ∘ ⇑isoD
gu : Du → Fu := ⇑isoF.symm ∘ g ∘ ⇑isoD
Bu₀ : Eu →L[𝕜] Fu →L[𝕜] G :=
  ((B.comp ↑{ toLinearEquiv := isoE.toLinearEquiv, continuous_toFun := ⋯, continuous_invFun := ⋯ }).flip.comp
      ↑{ toLinearEquiv := isoF.toLinearEquiv, continuous_toFun := ⋯, continuous_invFun := ⋯ }).flip
Bu : Eu →L[𝕜] Fu →L[𝕜] Gu :=
  ((compL 𝕜 Eu (Fu →L[𝕜] G) (Fu →L[𝕜] Gu))
      ((compL 𝕜 Fu G Gu) ↑{ toLinearEquiv := isoG.symm.toLinearEquiv, continuous_toFun := ⋯, continuous_invFun := ⋯ }))
    Bu₀
hBu :
  Bu =
    ((compL 𝕜 Eu (Fu →L[𝕜] G) (Fu →L[𝕜] Gu))
        ((compL 𝕜 Fu G Gu)
          ↑{ toLinearEquiv := isoG.symm.toLinearEquiv, continuous_toFun := ⋯, continuous_invFun := ⋯ }))
      Bu₀
Bu_eq : (fun y => (Bu (fu y)) (gu y)) = ⇑isoG.symm ∘ (fun y => (B (f y)) (g y)) ∘ ⇑isoD
Bu_le : ‖Bu‖ ≤ ‖B‖
su : Set Du := ⇑isoD ⁻¹' s
hsu : UniqueDiffOn 𝕜 su
xu : Du := isoD.symm x
hxu : xu ∈ su
xu_x : isoD xu = x
hfu : ContDiffOn 𝕜 (↑n) fu su
hgu : ContDiffOn 𝕜 (↑n) gu su
Nfu : ∀ (i : ℕ), ‖iteratedFDerivWithin 𝕜 i fu su xu‖ = ‖iteratedFDerivWithin 𝕜 i f s x‖
Ngu : ∀ (i : ℕ), ‖iteratedFDerivWithin 𝕜 i gu su xu‖ = ‖iteratedFDerivWithin 𝕜 i g s x‖
⊢ ‖iteratedFDerivWithin 𝕜 n (fun y => (B (f y)) (g y)) s x‖ ≤
    ‖B‖ *
      ∑ i ∈ Finset.range (n + 1),
        ↑(n.choose i) * ‖iteratedFDerivWithin 𝕜 i f s x‖ * ‖iteratedFDerivWithin 𝕜 (n - i) g s x‖