Masking snow AS

I’m still new to Flash and I have been unable to get this effect to work. I am using the snow actionscript found in the tutorial http://www.kirupa.com/developer/flash8/snow.htm
The actionscript for it is
[INDENT] init = function () { width = 300; // pixels height = 200; // pixels max_snowsize = 10; // pixels snowflakes = 50; // quantity for (i=0; i<snowflakes; i++) { t = attachMovie(“snow”, “snow”+i, i); t._alpha = 20+Math.random()60; t._x = -(width/2)+Math.random()(1.5width); t._y = -(height/2)+Math.random()(1.5height); t._xscale = t._yscale=50+Math.random()(max_snowsize10); t.k = 1+Math.random()2; t.wind = -1.5+Math.random()(1.43); t.onEnterFrame = mover;}}; mover = function() { this._y += this.k; this._x += this.wind; if (this._y>height+10) { this._y = -20;} if (this._x>width+20) { this._x = -(width/2)+Math.random()(1.5width); this._y = -20;} else if (this._x<-20) { this._x = -(width/2)+Math.random()(1.5width); this._y = -20;}} init(); [/INDENT]How can I add a mask to the flash so that the snow only falls behind the mask such as behind text? I have not been able to locate a topic that covers this and everything I have tried has not worked