setMask and path

hello,
I have a mask that is set by actionscript, the code is :
[AS]this.setMask(_root.contenu.photos.mask);
}[/AS]

But instead of _root, i’d like to use _parent because my movie is loaded into another one so i tried
[AS]this.setMask(_parent._parent._parent.mask);
}[/AS] but it doesn’t work, can someone tell me what is the good target ?
Thanks !
Ubik