reserve X for TopStruct,
  A for Subset of X;
reserve X for TopSpace,
  A,B for Subset of X;
reserve X for non empty TopSpace,
  A for Subset of X;
reserve X for TopSpace,
  A,B for Subset of X;

theorem Th11:
  B is boundary & A c= B implies A is boundary
by TOPS_1:19,XBOOLE_1:3;
