function createProdid() {
    if( !isset( $this->prodid ))
      $this->_makeProdid();
    switch( $this->format ) {
      case 'xcal':
        return ' prodid="'.$this->prodid.'"'.$this->nl;
        break;
      default:
        return 'PRODID:'.$this->prodid.$this->nl;
        break;
    }
  }