merging index entries

Hi,
I'm using the eclipse index.xsl code to generate a slightly customized xml format index. That is working fine, but the entries with identical primary terms need to be merged into a single entry.
For example, these two entries should be merged into one:
------------
<entry keyword="cakes">
  <topic href="ch01.html#intro">
  <entry keyword="chocolate">
    <topic href="ch01.html#choc">
  </entry>
</entry>
<entry keyword="cakes">
  <topic href="ch01.html#intro">
  <entry keyword="vanilla">
    <topic href="ch01.html#vnla">
  </entry>
------------
to look like this:
<entry keyword="cakes">
  <topic href="ch01.html#intro">
  <entry keyword="chocolate">
    <topic href="ch01.html#choc">
  </entry>
  <entry keyword="vanilla">
    <topic href="ch01.html#vnla">
  </entry>
</entry>


Can someone point me to some documentation or example code to do this merging?
thanks,
--Tim Arnold