∀ {X : Type u_1} [t : TopologicalSpace X] {C D : Set X} [inst : Topology.RelCWComplex C D] [T2Space X] {n : ℕ}
  {j : Topology.RelCWComplex.cell C n},
  closure (Topology.RelCWComplex.openCell n j) = Topology.RelCWComplex.closedCell n j