public static function allLeaves() {
    $instance = new static;

    $grammar = $instance->getConnection()->getQueryGrammar();

    $rgtCol = $grammar->wrap($instance->getQualifiedRightColumnName());
    $lftCol = $grammar->wrap($instance->getQualifiedLeftColumnName());

    return $instance->newQuery()
                    ->whereRaw($rgtCol . ' - ' . $lftCol . ' = 1')
                    ->orderBy($instance->getQualifiedOrderColumnName());
  }