Applying Masks, When Do They Work?

This should be an easy one for one of you flash hardcore elite. Basically i was wondering if it was possible to apply a mask over a scripted scene like for example creating the falling snow technique ([color=#800080]Falling Snow[/color] kirupa tutorial)[color=#006699][/color][color=#000000]and then applying a mask so the snow can be seen only through a window or something? I cant seem to make it work! [/color]

Also ive found that however high i put an item in the stack the snow always seems to be on top, is this the problem, is there a solution?

Any thoughts on this one apprechiated, dave…