[TYPO3] extending class.tslib_content

[Aji Johannes]

Hello List,

im trying to extend class.tslib_content with an extension.
i make extension, and want to extend "function editPanel":

i have in ext_localconf.php this code:

$TYPO3_CONF_VARS["FE"]["XCLASS"]["tslib/class.tslib_content.php"]= 
PATH_typo3conf."class.ux_tslib_content.php";


and in class.ux_tslib_content.php this code:

class ux_tslib_content extends tslib_content {
	
	function editPanel($content, $conf, $currentRecord='',$dataArr=array())	{
		
	}
	
}	

if (defined('FE') && 
$TYPO3_CONF_VARS[FE]['XCLASS']['tslib/class.tslib_content.php'])   {
 
include_once($TYPO3_CONF_VARS[FE]['XCLASS']['tslib/class.tslib_content.php']);
}


my question is:
am i doing right with :$TYPO3_CONF_VARS[FE]['XCLASS'] or mus be 
$TYPO3_CONF_VARS[TYPO3_MODE]['XCLASS'] ????

i hope that my question is clear
please i need your help

regards
Aji