[xsl] using xsl:result-document in a non-list situation

["Mark Wilson" <mark@xxxxxxxxxxxx>]

All of the examples for using <xsl:result-document> to produce multiple 
files (at least those that I've seen) write to a new file each time they 
encounter a new instance of a controlling element structure in a 'list' and 
use "position()" to help name the file. What technique do I apply to a 
document containing a <List> of <Items> such as the one shown below? Here 
the controlling element is not simply another <Year> element, but <Year> 
elements with differing content: 1998, 1999 and so on, with each year 
repeated within the source  document many times. Ideally, the output would 
be a series of files, one for each different <Year> content (1998.xml, 
1999.xml) and each sorted internally by <IssueNumber> and <Page>.

I have not been able to see how the stylesheet should be constructed. Any 
hints would be appreciated.
Thanks,
Mark

Source example:
<List>
   <Item>
       <Data>
           ...
       </Data>
       <Article>
           <Year>1999</Year>
           <IssueNumber>1</IssueNumber>
           <Page>6</Page>
       </Article>
   </Item>
   <Item>
       .......
   </Item>
</List>