/**
* Unset an instance of this class
*
* @access public
* @param PHPExcel $workbook Injected workbook identifying the instance to unset
*/
public static function unsetInstance(PHPExcel $workbook = NULL)
{
if ($workbook !== NULL) {
if (isset(self::$_workbookSets[$workbook->getID()])) {
unset(self::$_workbookSets[$workbook->getID()]);
}
}
}
