∀ (𝕜 : Type u_3) [inst : NontriviallyNormedField 𝕜] {E : Type u_1} [inst_1 : NormedAddCommGroup E]
  [inst_2 : NormedSpace 𝕜 E] {F : Type u_2} [inst_3 : NormedAddCommGroup F] [inst_4 : NormedSpace 𝕜 F]
  (f : OpenPartialHomeomorph E F) (n : WithTop ℕ∞),
  n ≠ ↑⊤ → IsOpen (f.source ∩ {x | ContDiffAt 𝕜 n (↑f) x ∧ ContDiffAt 𝕜 n (↑f.symm) (↑f x)})