Another addChild mindmelter

1

Please hava a look at this code:

 
public class Obj extends Sprite
 {
  public var _clip:Sprite;
  public function Obj(clip:Sprite=null)
  {
   this._clip = clip;
  }
  public function attachTo(scope:DisplayObjectContainer)
  {
   if (this._clip)
   {
    scope.addChild(this); // nothing is displayed
    // or scope.addChild(this._clip);
   }
  }
 }

Now i use

 
var obj = new Obj(new mc());
obj.attachTo(this.container_mc);

whereas the mc() is some library instance.
Naively i would have expected that if the _clip is a member of the Obj Class i would see it on stage when using scope.addChild(this).

But this is not the case.
I have to add the clip directly by using scope.addChild(this._clip) to see it being displayed. Why is that ??

Keith