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 Th26:
  Cl Int T = Cl Int Cl Int T
proof
  Int Int T c= Int Cl Int T by Th19,PRE_TOPC:18;
  then
A1: Cl Int T c= Cl Int Cl Int T by PRE_TOPC:19;
  Cl Int Cl Int T c= Cl Cl Int T by Th16,PRE_TOPC:19;
  hence thesis by A1;
end;
