∀ {𝕜 : Type u} [inst : NontriviallyNormedField 𝕜] {x : 𝕜} {s : Set 𝕜} {𝕜' : Type u_2} [inst_1 : NormedDivisionRing 𝕜']
  [inst_2 : NormedAlgebra 𝕜 𝕜'] {v : 𝕜 → 𝕜'} (u : 𝕜'), derivWithin (fun y => v y * u) s x = derivWithin v s x * u