Actionscript3 alpha masking?

Question

I was trying to apply a spotlight effect on a google map application. Specifically, I draw a circle that follows the mouse and set it as a mask over the map. The problem is

Answer 1

Make the mask much larger than it needs to be and fill it with some 'gray' so the map will show through and put the circle in the middle.

There's an example here: http://www.flashandmath.com/howtos/alphamask/

The key is the cacheAsBitmap = true, bit.