public function parenthesize_plural_exression( $expression ) {
      $expression .= ';';
      $res         = '';
      $depth       = 0;
      for ( $i = 0; $i < strlen( $expression ); ++$i ) {
        $char = $expression[ $i ];
        switch ( $char ) {
          case '?':
            $res .= ' ? (';
            ++$depth;
            break;
          case ':':
            $res .= ') : (';
            break;
          case ';':
            $res  .= str_repeat( ')', $depth ) . ';';
            $depth = 0;
            break;
          default:
            $res .= $char;
        }
      }
      return rtrim( $res, ';' );
    }