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;
reserve X for non empty TopSpace;

theorem Th40:
  for X1,X2 being SubSpace of X st X1 is nowhere_dense holds X2 is
  SubSpace of X1 implies X2 is nowhere_dense
by TSEP_1:4,Th39;
