public function __construct($rootName, $leafName, $isSortAscending) {
     parent::__construct($isSortAscending);
     $this->rootName = $rootName;
     $this->leafName = $leafName;
 }
 public function __construct($columnName, $isSortAscending = TRUE) {
     parent::__construct($isSortAscending);
     $this->columnName = $columnName;
 }