idref — Extract IDREF from a linking element
The idref template accepts 
any xref or universally linking element as a parameter
and returns the local
ID that it points to.
If it has a linkend attribute,
it returns that value.
If it has a xlink:href, it
returns that value, parsed as needed to extract just
the id reference.  Otherwise it returns an empty string.
<xsl:template name="idref">
  <xsl:param name="xref" select="."></xsl:param>
  <xsl:param name="linkend" select="$xref/@linkend"></xsl:param>
  <xsl:param name="xpointer">
    <xsl:call-template name="xpointer.idref">
      <xsl:with-param name="xpointer" select="$xref/@xlink:href"></xsl:with-param>
    </xsl:call-template>
  </xsl:param> 
  <xsl:choose>
    <xsl:when test="string-length($linkend) != 0">
      <xsl:value-of select="$linkend"></xsl:value-of>
    </xsl:when>
    <xsl:when test="string-length($xpointer) != 0">
      <xsl:value-of select="$xpointer"></xsl:value-of>
    </xsl:when>
    <!-- otherwise it's a pointer to some other document -->
  </xsl:choose>
</xsl:template>