Help with ClientBundle: neighbour @sprite offsets dependencies

[Vitrums]

I've got 5 sprites in a row. How can I get left offsets for the 3rd and 4th elements using early binding? I'm trying to do something like calculate a sum of 1st and 2nd images widths, using value('imageResource.getWidth','px') function or anything of that kind... but didn't manage to find any solution to evaluate those expressions( If there was no workaround, would there at least be a way to implement some custom generator as a plug-in over proprietary GWT css processing facility?