code: public AggGroupSet addGroupSet() {
          AggGroupSet aggGroupSet = new AggGroupSet();
          if (m_groupSetList == null) {
              m_groupSetList = new ArrayList<AggGroupSet>();
          }
          m_groupSetList.add(aggGroupSet);
          return aggGroupSet;
      }
docs: Create and return a new {@link AggGroupSet} that belongs to this AggregateResult.
  
  @return  A new {@link AggGroupSet} that belongs to this AggregateResult.