reserve A, B for non empty set,
  A1, A2, A3 for non empty Subset of A;
reserve X for TopSpace;
reserve X for non empty TopSpace;
reserve X1, X2 for non empty SubSpace of X;
reserve X0, X1, X2, Y1, Y2 for non empty SubSpace of X;

theorem
  X0 union X0 = the TopStruct of X0
proof
  X0 is SubSpace of X0 by TSEP_1:2;
  hence thesis by TSEP_1:23;
end;
