∀ {𝕜 : Type u} [inst : NontriviallyNormedField 𝕜] {x : 𝕜} {𝔸 : Type u_3} [inst_1 : NormedRing 𝔸]
  [inst_2 : NormedAlgebra 𝕜 𝔸] {c d : 𝕜 → 𝔸},
  DifferentiableAt 𝕜 c x → DifferentiableAt 𝕜 d x → deriv (c * d) x = deriv c x * d x + c x * deriv d x