Grouping on multiple nested elements

Dear all,

I have XML data like 

<?xml version="1.0"?>
<data>
<story>
  <story_id>589608</story_id>
  <story_title>It's Only Human Nature</story_title>
  <program_name>Science Show</program_name>
  <transmission_date>20020629</transmission_date>
  <description>What is human nature? Who really knows?</description>
  <subjects>
    <subject>Psychology</subject>
    <subject>Health</subject>
    <subject></subject>
    <subject></subject>
    <subject></subject>
    <subject></subject>
    <subject></subject>
    <subject></subject>
  </subjects>
</story>
<story>
  <story_id>591325</story_id>
  <story_title>Fins responsible for Hyshot crash</story_title>
  <program_name>Science News</program_name>
  <transmission_date>20020627</transmission_date>
  <description>The failure of a set of fins on the launch rocket was likely
to be the cause of the Hyshot scramjet crash in October 2001, according to
the final report of the investigation released last week. The finding
clears the way for a second launch.</description>
  <subjects>
    <subject>Australian</subject>
    <subject>Space</subject>
    <subject>Engineering</subject>
    <subject></subject>
    <subject></subject>
    <subject></subject>
    <subject></subject>
    <subject></subject>
  </subjects>
</story>
<story>
  <story_id>591756</story_id>
  <story_title>Immunisation gets bad rap on internet</story_title>
  <program_name>Science News</program_name>
  <transmission_date>20020627</transmission_date>
  <description>The internet has an abundance of websites that are negative
towards vaccination and tend to rely on unscientific evidence according to
two separate studies published this week.</description>
  <subjects>
    <subject>Australian</subject>
    <subject>Health</subject>
    <subject>Information Technology</subject>
    <subject></subject>
    <subject></subject>
    <subject></subject>
    <subject></subject>
    <subject></subject>
  </subjects>
</story>
</data>

(but more) and I am attempting to output stories grouped by their
subject(s) e.g.

Australian

	story title
	description

	story title
	description

	...

Health

	story title
	description

	...

Space

	...


I have no problem grouping by program_name using the Muenchian method -
there is only one program_name for each story. However, grouping by subject
has me stumped - do I need multiple keys and how do I keep track of the
current subject?

Apologies if this has been covered previously but I have search the
archives in vain.

Regards
Paul Dickinson
ABC Online 



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