docs: public DBag difference(DBag otherBag)
      {
          DBagImpl result = new DBagImpl(getPBKey());
          Iterator iter = this.iterator();
          while (iter.hasNext())
          {
              Object candidate = iter.next();
              if (!otherBag.contains(candidate))
              {
                  result.add(candidate);
              }
          }
          return result;
      }