removeChild Only if Something Is There Already

programming flash
1

I have some buttons that are loading external SWF files into a movie clip called holder. Each button loads SWFs into that clip, but the addChild method puts one on top of the other in sequence. How can I use removeChild to remove specifically, the content of the holder clip?

I have seen this: if (this.contains(ball)) this.removeChild(ball); But it doesn’t serve my purposes because the movie clip is a placeholder. Here’s my code:

b1.addEventListener(MouseEvent.CLICK, swf1);
function swf1(event:MouseEvent):void {
	var swf1Ldr:Loader = new Loader();
	var swf1URL:String = "dieHard.swf";
	var swf1URLReq:URLRequest = new URLRequest(swf1URL);
	swf1Ldr.load(swf1URLReq);
	swf1Ldr.contentLoaderInfo.addEventListener(Event.COMPLETE, swf1Loaded);
	function swf1Loaded(event:Event):void {
		holder.addChild(swf1Ldr.content);
	}
	// Make the progress clip visible and show the progress loaded
	swf1Ldr.contentLoaderInfo.addEventListener(ProgressEvent.PROGRESS, showProgress);

}

Any ideas