∀ {𝕜 : Type u} [inst : NontriviallyNormedField 𝕜] {F : Type v} [inst_1 : NormedAddCommGroup F]
  [inst_2 : NormedSpace 𝕜 F] {x : 𝕜} {ι : Type u_1} {u : Finset ι} {A : ι → 𝕜 → F} {A' : ι → F},
  (∀ i ∈ u, HasStrictDerivAt (A i) (A' i) x) → HasStrictDerivAt (fun y => ∑ i ∈ u, A i y) (∑ i ∈ u, A' i) x