/-- If a function f has a derivative f' at x, a rescaled version of f around x converges to f',
i.e., `n (f (x + (1/n) v) - f x)` converges to `f' v`. More generally, if `c n` tends to infinity
and `c n * d n` tends to `v`, then `c n * (f (x + d n) - f x)` tends to `f' v`. This lemma expresses
this fact, for functions having a derivative within a set. Its specific formulation is useful for
tangent cone related discussions. -/
theorem HasFDerivWithinAt.lim (h : HasFDerivWithinAt f f' s x) {α : Type*} (l : Filter α)
    {c : α → 𝕜} {d : α → E} {v : E} (dtop : ∀ᶠ n in l, x + d n ∈ s)
    (clim : Tendsto (fun n => ‖c n‖) l atTop) (cdlim : Tendsto (fun n => c n • d n) l (𝓝 v)) :
    Tendsto (fun n => c n • (f (x + d n) - f x)) l (𝓝 (f' v)) := by
  have tendsto_arg : Tendsto (fun n => x + d n) l (𝓝[s] x) := by
    conv in 𝓝[s] x => rw [← add_zero x]
    rw [nhdsWithin, tendsto_inf]
    constructor
    · apply tendsto_const_nhds.add (tangentConeAt.lim_zero l clim cdlim)
    · rwa [tendsto_principal]
  have : (fun y => f y - f x - f' (y - x)) =o[𝓝[s] x] fun y => y - x := h.isLittleO
  have : (fun n => f (x + d n) - f x - f' (x + d n - x)) =o[l] fun n => x + d n - x :=
    this.comp_tendsto tendsto_arg
  have : (fun n => f (x + d n) - f x - f' (d n)) =o[l] d := by simpa only [add_sub_cancel']
  have : (fun n => c n • (f (x + d n) - f x - f' (d n))) =o[l] fun n => c n • d n :=
    (isBigO_refl c l).smul_isLittleO this
  have : (fun n => c n • (f (x + d n) - f x - f' (d n))) =o[l] fun _ => (1 : ℝ) :=
    this.trans_isBigO (cdlim.isBigO_one ℝ)
  have L1 : Tendsto (fun n => c n • (f (x + d n) - f x - f' (d n))) l (𝓝 0) :=
    (isLittleO_one_iff ℝ).1 this
  have L2 : Tendsto (fun n => f' (c n • d n)) l (𝓝 (f' v)) :=
    Tendsto.comp f'.cont.continuousAt cdlim
  have L3 :
    Tendsto (fun n => c n • (f (x + d n) - f x - f' (d n)) + f' (c n • d n)) l (𝓝 (0 + f' v)) :=
    L1.add L2
  have :
    (fun n => c n • (f (x + d n) - f x - f' (d n)) + f' (c n • d n)) = fun n =>
      c n • (f (x + d n) - f x) := by
    

Proof state:
𝕜 : Type u_1
inst✝⁸ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁷ : NormedAddCommGroup E
inst✝⁶ : NormedSpace 𝕜 E
F : Type u_3
inst✝⁵ : NormedAddCommGroup F
inst✝⁴ : NormedSpace 𝕜 F
G : Type u_4
inst✝³ : NormedAddCommGroup G
inst✝² : NormedSpace 𝕜 G
G' : Type u_5
inst✝¹ : NormedAddCommGroup G'
inst✝ : NormedSpace 𝕜 G'
f f₀ f₁ g : E → F
f' f₀' f₁' g' e : E →L[𝕜] F
x : E
s t : Set E
L L₁ L₂ : Filter E
h : HasFDerivWithinAt f f' s x
α : Type u_6
l : Filter α
c : α → 𝕜
d : α → E
v : E
dtop : ∀ᶠ (n : α) in l, x + d n ∈ s
clim : Tendsto (fun n => ‖c n‖) l atTop
cdlim : Tendsto (fun n => c n • d n) l (𝓝 v)
tendsto_arg : Tendsto (fun n => x + d n) l (𝓝[s] x)
this✝³ : (fun y => f y - f x - f' (y - x)) =o[𝓝[s] x] fun y => y - x
this✝² : (fun n => f (x + d n) - f x - f' (x + d n - x)) =o[l] fun n => x + d n - x
this✝¹ : (fun n => f (x + d n) - f x - f' (d n)) =o[l] d
this✝ : (fun n => c n • (f (x + d n) - f x - f' (d n))) =o[l] fun n => c n • d n
this : (fun n => c n • (f (x + d n) - f x - f' (d n))) =o[l] fun x => 1
L1 : Tendsto (fun n => c n • (f (x + d n) - f x - f' (d n))) l (𝓝 0)
L2 : Tendsto (fun n => f' (c n • d n)) l (𝓝 (f' v))
L3 : Tendsto (fun n => c n • (f (x + d n) - f x - f' (d n)) + f' (c n • d n)) l (𝓝 (0 + f' v))
⊢ (fun n => c n • (f (x + d n) - f x - f' (d n)) + f' (c n • d n)) = fun n => c n • (f (x + d n) - f x)

Next tactic:
ext n