Fill or swatch that makes layers underneath transparent?

[DivideByIchi]

I'm working on a vector with Illustrator CS3, and soon I'm planning to have many of the elements animate. What I'm looking for is a swatch or something that I can fill a shape with that will occlude everything on every layer underneath that shape.

To explain further, let's say I have (in ascending order) two layers. Layer 1 is filled entirely with yellow. Layer 2 has a shape of a circle with a black stroke and no fill. Since it has no fill, the yellow can be seen through the inside of the circle. What I want to do is turn the circle into something akin to a window or an X-ray device by using something (what it is, I'm not sure) that, if I fill the circle with it, will let me see through the yellow on Layer 1. And I want to be able to do this without permanently or "physically" altering Layer 1 (such as selecting the innards of the circle, switching to Layer 1, and deleting the selection), so that I can move the circle around and it will remain occluding the objects underneath.

Does such a thing exist? Or are there any effective alternatives that I can use? If so, I'd be most grateful for your advice.

 

[jconingham]

The best way to do this would be to set up a mask in Flash (assuming that's what you are going to use for the animation).