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

   </b>
          <b>
            <c>3</c>
            <d>6</d>
        sum() applied to a product

 
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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?