∀ {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] (f : E → F)
  {f' : E ≃L[𝕜] F} (s : Set E) {c : NNReal} [inst_5 : CompleteSpace E] (hf : ApproximatesLinearOn f (↑f') s c)
  (hc : Subsingleton E ∨ c < ‖↑f'.symm‖₊⁻¹) (hs : IsOpen s),
  (ApproximatesLinearOn.toOpenPartialHomeomorph f s hf hc hs).source = s