-- Lean 4 proof step from Mathlib.Analysis.Calculus.ContDiff.FaaDiBruno (goal)
Goal:
𝕜 : Type u_1
inst : NontriviallyNormedField 𝕜
E : Type u_2
inst_1 : NormedAddCommGroup E
inst_2 : NormedSpace 𝕜 E
F : Type u_3
inst_3 : NormedAddCommGroup F
inst_4 : NormedSpace 𝕜 F
G : Type u_4
inst_5 : NormedAddCommGroup G
inst_6 : NormedSpace 𝕜 G
s : Set E
t : Set F
q : F → FormalMultilinearSeries 𝕜 F G
p : E → FormalMultilinearSeries 𝕜 E F
n : WithTop ℕ∞
g : F → G
f : E → F
hg : HasFTaylorSeriesUpToOn n g q t
hf : HasFTaylorSeriesUpToOn n f p s
h : MapsTo f s t
zero_eq : ∀ x ∈ s, ContinuousMultilinearMap.curry0 ((q (f x)).taylorComp (p x) 0) = (g ∘ f) x
⊢ HasFTaylorSeriesUpToOn.{u_1, u_2, u_4} (𝕜 := 𝕜) n (g ∘ f) (fun (x : E) ↦ (q (f x)).taylorComp (p x)) s
have fderivWithin : ∀ (m : ℕ),
  (↑m : WithTop ℕ∞) < n →
    ∀ x ∈ s,
      HasFDerivWithinAt (F := ContinuousMultilinearMap.{u_1, 0, u_2, u_4} 𝕜 (fun (i : Fin m) ↦ E) G)
        (fun (x : E) ↦ (q (f x)).taylorComp (p x) m)
        (ContinuousMultilinearMap.curryLeft (Ei := fun (i : Fin m.succ) ↦ E) ((q (f x)).taylorComp (p x) m.succ)) s x
Tactic: classical
  constructor
  · intro x hx
    simp [FormalMultilinearSeries.taylorComp, default, HasFTaylorSeriesUpToOn.zero_eq' hg (h hx)]
  · intro m hm x hx
    have A (c : OrderedFinpartition m) :
      HasFDerivWithinAt (fun x ↦ (q (f x)).compAlongOrderedFinpartition (p x) c)
        (∑ i : Option (Fin c.length),
          ((q (f x)).compAlongOrderedFinpartition (p x) (c.extend i)).curryLeft) s x := by
      let B := c.compAlongOrderedFinpartitionL 𝕜 E F G
      change HasFDerivWithinAt (fun y ↦ B (q (f y) c.length) (fun i ↦ p y (c.partSize i)))
        (∑ i : Option (Fin c.length),
          ((q (f x)).compAlongOrderedFinpartition (p x) (c.extend i)).curryLeft) s x
      have cm : (c.length : WithTop ℕ∞) ≤ m := mod_cast OrderedFinpartition.length_le c
      have cp i : (c.partSize i : WithTop ℕ∞) ≤ m := by
        exact_mod_cast OrderedFinpartition.partSize_le c i
      have I i : HasFDerivWithinAt (fun x ↦ p x (c.partSize i))
          (p x (c.partSize i).succ).curryLeft s x :=
        hf.fderivWithin (c.partSize i) ((cp i).trans_lt hm) x hx
      have J : HasFDerivWithinAt (fun x ↦ q x c.length) (q (f x) c.length.succ).curryLeft
        t (f x) := hg.fderivWithin c.length (cm.trans_lt hm) (f x) (h hx)
      have K : HasFDerivWithinAt f ((continuousMultilinearCurryFin1 𝕜 E F) (p x 1)) s x :=
        hf.hasFDerivWithinAt hm.ne_bot hx
      convert! HasFDerivWithinAt.linear_multilinear_comp (J.comp x K h) I B
      simp only [B, Nat.succ_eq_add_one, Fintype.sum_option, comp_apply, faaDiBruno_aux1,
        faaDiBruno_aux2]
    have B : HasFDerivWithinAt (fun x ↦ (q (f x)).taylorComp (p x) m)
        (∑ c : OrderedFinpartition m, ∑ i : Option (Fin c.length),
          ((q (f x)).compAlongOrderedFinpartition (p x) (c.extend i)).curryLeft) s x :=
      HasFDerivWithinAt.fun_sum (fun c _ ↦ A c)
    suffices ∑ c : OrderedFinpartition m, ∑ i : Option (Fin c.length),
          ((q (f x)).compAlongOrderedFinpartition (p x) (c.extend i)) =
        (q (f x)).taylorComp (p x) (m + 1) by
      rw [← this]
      convert! B
      ext v
      simp only [Nat.succ_eq_add_one, Fintype.sum_option, ContinuousMultilinearMap.curryLeft_apply,
        ContinuousMultilinearMap.sum_apply, ContinuousMultilinearMap.add_apply,
        FormalMultilinearSeries.compAlongOrderedFinpartition_apply, ContinuousLinearMap.coe_sum',
        Finset.sum_apply, ContinuousLinearMap.add_apply]
    rw [Finset.sum_sigma']
    exact Fintype.sum_equiv (OrderedFinpartition.extendEquiv m) _ _ (fun p ↦ rfl)
  · intro m hm
    apply continuousOn_finsetSum _ (fun c _ ↦ ?_)
    let B := c.compAlongOrderedFinpartitionL 𝕜 E F G
    change ContinuousOn
      ((fun p ↦ B p.1 p.2) ∘ (fun x ↦ (q (f x) c.length, fun i ↦ p x (c.partSize i)))) s
    apply B.continuous_uncurry_of_multilinear.comp_continuousOn (ContinuousOn.prodMk ?_ ?_)
    · have : (c.length : WithTop ℕ∞) ≤ m := mod_cast OrderedFinpartition.length_le c
      exact (hg.cont c.length (this.trans hm)).comp hf.continuousOn h
    · apply continuousOn_pi.2 (fun i ↦ ?_)
      have : (c.partSize i : WithTop ℕ∞) ≤ m := by
        exact_mod_cast OrderedFinpartition.partSize_le c i
      exact hf.cont _ (this.trans hm)