reserve TS for 1-sorted,
  K, Q for Subset of TS;
reserve TS for TopSpace,
  GX for TopStruct,
  x for set,
  P, Q for Subset of TS,
  K , L for Subset of TS,
  R, S for Subset of GX,
  T, W for Subset of GX;

theorem
  Fr Cl T c= Fr T
proof
  T c= Cl T by PRE_TOPC:18;
  then (Cl T)` c= T` by SUBSET_1:12;
  then Cl((Cl T)`) c= Cl(T`) by PRE_TOPC:19;
  hence thesis by XBOOLE_1:26;
end;
