-- Mathlib/Analysis/Calculus/ContDiff/Defs.lean
theorem contDiffOn_of_continuousOn_differentiableOn {n : ℕ∞}
    (Hcont : ∀ m : ℕ, m ≤ n → ContinuousOn (fun x => iteratedFDerivWithin 𝕜 m f s x) s)
    (Hdiff : ∀ m : ℕ, m < n →
      DifferentiableOn 𝕜 (fun x => iteratedFDerivWithin 𝕜 m f s x) s) :
    ContDiffOn 𝕜 n f s

Proof state:
case fderivWithin
𝕜 : Type u
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type uE
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type uF
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
s : Set E
f : E → F
n : ℕ∞
Hcont : ∀ (m : ℕ), ↑m ≤ n → ContinuousOn (fun x => iteratedFDerivWithin 𝕜 m f s x) s
Hdiff : ∀ (m : ℕ), ↑m < n → DifferentiableOn 𝕜 (fun x => iteratedFDerivWithin 𝕜 m f s x) s
x : E
hx : x ∈ s
m : ℕ
hm : ↑m ≤ n
⊢ ∀ (m_1 : ℕ),
    ↑m_1 < ↑m →
      ∀ x ∈ s,
        HasFDerivWithinAt (fun x => ftaylorSeriesWithin 𝕜 f s x m_1) (ftaylorSeriesWithin 𝕜 f s x m_1.succ).curryLeft s
          x

Tactic:
intro k hk y hy

Result:
case fderivWithin
𝕜 : Type u
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type uE
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type uF
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
s : Set E
f : E → F
n : ℕ∞
Hcont : ∀ (m : ℕ), ↑m ≤ n → ContinuousOn (fun x => iteratedFDerivWithin 𝕜 m f s x) s
Hdiff : ∀ (m : ℕ), ↑m < n → DifferentiableOn 𝕜 (fun x => iteratedFDerivWithin 𝕜 m f s x) s
x : E
hx : x ∈ s
m : ℕ
hm : ↑m ≤ n
k : ℕ
hk : ↑k < ↑m
y : E
hy : y ∈ s
⊢ HasFDerivWithinAt (fun x => ftaylorSeriesWithin 𝕜 f s x k) (ftaylorSeriesWithin 𝕜 f s y k.succ).curryLeft s y

Proof state:
𝕜 : Type u
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type uE
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type uF
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
s : Set E
f : E → F
n : ℕ∞
Hcont : ∀ (m : ℕ), ↑m ≤ n → ContinuousOn (fun x => iteratedFDerivWithin 𝕜 m f s x) s
Hdiff : ∀ (m : ℕ), ↑m < n → DifferentiableOn 𝕜 (fun x => iteratedFDerivWithin 𝕜 m f s x) s
x : E
hx : x ∈ s
m : ℕ
hm : ↑m ≤ n
⊢ ∃ u ∈ 𝓝[insert x s] x, ∃ p, HasFTaylorSeriesUpToOn (↑m) f p u

Tactic:
rw [insert_eq_of_mem hx]

Result:
𝕜 : Type u
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type uE
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type uF
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
s : Set E
f : E → F
n : ℕ∞
Hcont : ∀ (m : ℕ), ↑m ≤ n → ContinuousOn (fun x => iteratedFDerivWithin 𝕜 m f s x) s
Hdiff : ∀ (m : ℕ), ↑m < n → DifferentiableOn 𝕜 (fun x => iteratedFDerivWithin 𝕜 m f s x) s
x : E
hx : x ∈ s
m : ℕ
hm : ↑m ≤ n
⊢ ∃ u ∈ 𝓝[s] x, ∃ p, HasFTaylorSeriesUpToOn (↑m) f p u

Proof state:
𝕜 : Type u
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type uE
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type uF
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
s : Set E
f : E → F
n : ℕ∞
Hcont : ∀ (m : ℕ), ↑m ≤ n → ContinuousOn (fun x => iteratedFDerivWithin 𝕜 m f s x) s
Hdiff : ∀ (m : ℕ), ↑m < n → DifferentiableOn 𝕜 (fun x => iteratedFDerivWithin 𝕜 m f s x) s
⊢ ContDiffOn 𝕜 (↑n) f s

Tactic:
intro x hx m hm

Result:
𝕜 : Type u
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type uE
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type uF
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
s : Set E
f : E → F
n : ℕ∞
Hcont : ∀ (m : ℕ), ↑m ≤ n → ContinuousOn (fun x => iteratedFDerivWithin 𝕜 m f s x) s
Hdiff : ∀ (m : ℕ), ↑m < n → DifferentiableOn 𝕜 (fun x => iteratedFDerivWithin 𝕜 m f s x) s
x : E
hx : x ∈ s
m : ℕ
hm : ↑m ≤ n
⊢ ∃ u ∈ 𝓝[insert x s] x, ∃ p, HasFTaylorSeriesUpToOn (↑m) f p u