It doesn't appear that I can do:

mc.setMask(mask1);
mc.setMask(mask2);
mc.setMask(mask3);

(the last mask is the only one that will work)

I'm trying to create a kind of Venetian blinds effect...There is a grid of movieclips, each clip has a front and a back. At a mouse click, the front does a rotating card flip effect, and I want the backside of each clip to be a mask for a larger movieclip.