docs: public Set<String> getGroupIds( String parentGroupId )
      {
          Set<String> result = new TreeSet<String>();
          for ( int i = 0; i < stores.length; i++ )
          {
              Set<String> groupIds = stores[i].getGroupIds( parentGroupId );
              if ( groupIds != null )
              {
                  result.addAll( groupIds );
              }
          }
          return result;
      }