static private function translate ($expression)
  {
    list ($main, $op) = self::translateSimpleExpression ($expression);

    $exp = $main;
    if ($op == '|') {
      $filters = preg_split (self::PARSE_FILTER, $expression);
      if ($filters)
        foreach ($filters as $filter)
          $exp = self::translateFilter ($filter, $exp);
    }

    return $exp;
  }