reserve X for non empty TopSpace,
  A,B for Subset of X;
reserve Y1,Y2 for non empty SubSpace of X;
reserve X1, X2 for non empty SubSpace of X;

theorem Th29:
  for X0 being SubSpace of X, A being Subset of X st A = the
  carrier of X0 holds X0 is boundary iff A is boundary;
