i´m trying to place a loader content (an swf which is an instance of the class "Nav") into a movieclip (so i can easily refer to its variables, for example, width) but when i trace the movieclip it throws null. why ?

Code:
		private function doneNav(e:Event):void
		{
			trace (navLoader.content);	// [Object Nav].  fine!
			nav = navLoader.content as MovieClip;	// null.   why??
			
			trace (navLoader.getChildAt(0));	// [Object Nav].  fine!
			nav = navLoader.getChildAt(0) as MovieClip;	// null.   why??
i´d like to later in the code do this:

Code:
			nav.x = (stage.stageWidth - nav.width)/2;
what are the best practices in loading swfs?

thanks!