∀ {X : Type u_1} [t : TopologicalSpace X] {C D : Set X} [T2Space X] [inst : Topology.RelCWComplex C D]
  (E : Topology.RelCWComplex.Subcomplex C) {n : ℕ} {i : Topology.RelCWComplex.cell C n},
  i ∈ E.I n → Topology.RelCWComplex.cellFrontier n i ⊆ ↑E