attachMovie does not unload whatever's on level 0

in my movieclip, I have some objects that I have manually inserted (at design time). Then at run-time I call:

mc.attachMovie(“box”,“box1”,0)

note that I specify level 0. However, all the objects in the movieclip are still there. I thought they were supposed to be replaced, since they lie on level 0. Is that supposed to happen?

So how can I delete whatever’s in this movieclip? I also tried mc.clear() but it doesn’t work either. Thanks!