Mask in "falling snow tutorial"

[pfphoto]

Hello:
I,m using flash mx, and I,m trying to use the tutorial of this page "Falling Snow" in the Special Effects section.
I want to know if its posible to set a mask for an specifict area of the image, because I dont want to show the snow in all the movie, only in an area.
I don,t know so much of actionscript.
Somebody can help me.
Thanks in advance and sorry for my english.

pfphoto from Spain

[pfphoto]

Sorry, no reply, I found the solutíon. I put the movieclip of the snow inside another mc, and in this way it,s posible to mask it.
Thanks and sorry