sum() applied to a product

[Brian Chrisman <brian.chrisman@xxxxxx>]

Uggg... I had some sloppy cut-paste issue in my last post... this is what it should have been, with a real subject line.  Sorry...

-----------------------
Okay.. I told someone I could do this in a simple xpath expression

<?xml version="1.0"?>
<a>
 <b>
   <c>2</c>
   <d>4</d>
 </b>
 <b>
   <c>3</c>
   <d>6</d>
 </b>
</a>

Where the xpath was to return (2*4) + (3*6) = 26
I made several attempts with xpath's sum() function (using 1.0) and 
couldn't get it.
I couldn't even come up with an expression that I thought might work.. 
even though I tried several which I was certain would fail (which did).

Any ideas?