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 Th34:
  for X1,X2 being SubSpace of X st X1 is boundary holds X2 is
  SubSpace of X1 implies X2 is boundary
by TSEP_1:4,Th33;
