wishlist: expressions in fields

[Tripp Lilley]

I'd like to be able to do spreadsheet-like expressions in fields as I'm
entering things. For instance, in the absence of a cool "do sales tax"
button, I'd like to be able to enter .045 * (8.96 + 7.67 + 7.97) or,
better yet, .045 * sum([1],[3]) (where "1" and "3" are references to split
rows).

Now, more specifically, obviously, I'm trying to enter sales tax on
itemized bits :) So it would be wicked cool to be able to flag each split
row as taxable (food / merchandise at different rates), and have gnc do
the math for me, without even my entering an expression.

But in the short term, a basic expressions-in-fields facility, with some
syntax for referring to split rows / fields would be wicked cool.