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Oh, another way of explaing this is:

I would like to know a method that will take two identical (!) semi-transparent images consisting only of the color #000000 and subtracting one from the other leaving nothing at all.

I think the solution has to do with alpha layers - or possibly clipping masks; I never really understood their function. In any case, using regular masks could only ever knock out part of the opacity. Maybe if I created the original images using masks in the first place ... but then I guess the question would be more like: which method subtracts two identical greyscale images leaving nothing.