∀ {𝕜 : Type u} [inst : NontriviallyNormedField 𝕜] {F : Type v} [inst_1 : NormedAddCommGroup F]
  [inst_2 : NormedSpace 𝕜 F] {f g : 𝕜 → F} {x : 𝕜},
  DifferentiableAt 𝕜 f x → DifferentiableAt 𝕜 g x → deriv (f - g) x = deriv f x - deriv g x