XPath question

Every time I think I have a good grasp on XPath, it slips between my fingers. 
Consider the following XML:

<menu>
  <item title="Vegetables">
    <item title="tubers">
      <item title="potatoes">
        <item>Idaho</item>
        <item>Maine</item>
        <item>Russet</item>
        <item>Yam</item>
      </item>
    </item>
  </item>
  <item title="Fruits">
    <item title="Apples>
      <item>Fuji</item>
      <item>Granny Smith</item>
      <item>Rome</item>
  </item>
  <item>Milk</item>
</menu>

I have an HTML application for which I must flatten out the hierachy of this 
XML document, yet still maintain a link to the parent element for each child 
element. To this end I have decided to number each element according to its 
position in the hierarachy and use a "dot-delimitted" format for each element's 
id (i.01.03.11). I have struggled in vain today to come up with an XPath 
expression which would tell me how many elements are on the preceding-sibling 
axis of the parent of a given node. Can anyone straighten me out on this?

Thanks.
-- 
Charles Knell
cknell(_at_)onebox(_dot_)com - email

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