[xsl] Sorting by child element count

["Mark" <mark@xxxxxxxxxxxx>]

Happy new year! Thanks for all the help this list has given me over the past 
two years. My question:

I have an XML document that looks like the listing below. I want to sort the 
<List> by <Item> so that all the <Items> with one <Data> element are 
followed by all the <Items> with two <Data> elements, and so on, until the 
last <Items> in the output list have the most <Data> elements. The <Data> 
elements have to remain inside their original parent <Item>.

No clue how to begin. Can you point me in the right direction?

Thanks,
Mark

<List>
   <Item>
       <Data>...</Data>
           ... <!bpossibly 1 to 10 or so <Data> elements in each parent 
<Item> element.
       <Data>...</Data>
   </Item>
    ... <!b400 <Item> elements in the <List> b>
</List>