Subtracting two images in a filter

Hi all

I've been experimenting with a filter that requires that I subtract two
inputs. I might be missing something, but there doesn't seem to be a way to
do that.

The closest you can get, that I can see, is with <feComposite>

    <feComposite operator="arithmetic" in="a" in2="b" k2="1" k3="-1"/>

But because the arithmetic applies to all channels, the alpha channels
cancel out and you get a blank result (1-1=0).

It seems as if it would be useful if feComposite had a way to preserve the
alpha, similar to feConvolveMatrix.

Does this idea make sense?  Is it worth creating a feature request to
discuss this idea?

Paul