[filter-effects] additive animation of filter functions

During the SVG telcon this morning, we talked about what it would mean 
to allow additive animation of filter functions in the 'filter' 
property, and I said I would send a mail with some more details (this is 
my ACTION-3520).

Remember that "additive animations" in SVG are those that layer on top 
of other animations for the same attribute/property.  So if you had:

<g>
   <animate attributeName="filter" dur="10s"
            values="grayscale(0.5) blur(0px); grayscale(1) blur(4px)"/>
   <animate attributeName="filter" dur="10s"
            values="opacity(1); opacity(0.5)" additive="sum"/>
</g>

what is the animated value of 'filter' at a given point during the 
animation?

I think it makes sense for the "adding" operation to be "concatenate the 
lists".  So the value at t=5s would be:

   grayscale(0.75) blur(2px) opacity(0.75)