recursive addition

I have a structure (much like a file system) where elements of the same type are linked together via IDs rather than the XML structure--i.e. all directory elements are at the same level.

For an arbitrary directory level element, I want to sum a numeric attribute for all linked elements and so on down the tree.

Is there any way to do this?  I know that recursion is the first step, but what I don't know how to sum an arbitrary number of parallel level attributes in different elements.

Here's an example:
<root>
   <folder files="2" id="dir0">
      <file id="dir1" />

      <file id="dir4" />
   </folder>

   <folder files="4" id="dir1">
      <file id="dir2" />

      <file id="dir3" />
   </folder>

   <folder files="3" id="dir2" />

   <folder files="6" id="dir3" />

   <folder files="7" id="dir4" />

   <folder files="5" id="dir5" />
</root>

If my arbitrary element id is "dir1", then the result should be 9. (folder[@id='dir2']/@files + folder[@id='dir3']/@files)

How do I do that with xsl?


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