wish: built-in calculator in Amount field

Hi,

Here is feature request which is extremely important for efficient inserting split transactions.
Let say I have a receipt from supermarket:

Bread 3$
Cheese 2$
Ham 5$
Toy 10$
Puzzles 7$
-----------
Total 27$

As you can see, I have there articles from two categories:
 - Groceries
 - Articles for Children

Now I'd like to insert new Split Transaction. I'm choosing category Groceries, and hit twice TAB to focus in Amount field. Now I'd like to sum all of the articles from the same category in the field just by typing: "3+2+5", and program could automatically convert the string into 10$.

Then I add the second category, hit 2x<TAB> and hit "10+7", and <ENTER>.

All done. Much, MUCH faster than adding each article separately or summing them in different place.