grouping problem

Hi Folks,
           I am just trying to get the unique values
of  an attribute and just not getting it. I had
succcess in getting the unique values for elements
with the same expression.Am i missing something or is
there a different way to get the unique values for
attributes

<xsl:for-each select="//conept/@cid[not(. =
following::concept/@cid)]">
<xsl:value-of select="."/>
</xsl:for-each>

The above statement doesnot give me the unique values
of the @cid attribute.

Thanks
Thomas

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