select distinct

I have an xml file

<document>
 <title>Title</title>
 <section name="summary">
  <title>Executive Summary</title>
  <para>Some text goes here</para>
 </section>
 <section>
  <title>Detail</title>
  <para>Some more text goes here <ref name="a1" />.  some more text</para>
 </section>
 <section>
  <para>More text <ref name="a2" /> <ref name="a1" /></para>
 </section>
</document>

I would like to get all the unique <ref /> elements. 

<ref name="a1" />
<ref name="a2" />

I am not sure what my xsl should look like to accomplish this.

Thanks
Noel


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