code: public static ModifiableDBIDs difference(DBIDs ids1, DBIDs ids2) {
      ModifiableDBIDs result = DBIDUtil.newHashSet(ids1);
      result.removeDBIDs(ids2);
      return result;
    }
docs: Returns the difference of the two specified collection of IDs.
  
  @param ids1 the first collection
  @param ids2 the second collection
  @return the difference of ids1 minus ids2