I recently wanted to deal with docbook <ulink> elements that didn't have any contents by displaying the url as the link text. I wanted to convert:

<ulink url="http://www.example.com">Example.com<ulink>
<ulink url="http://www.example.com"/>

to

<a href="http://www.example.com">Example.com<a>
<a href="http://www.example.com">http://www.example.com<a>

I originally had:

<xsl:template match="ulink">
   <xsl:element name="a">
      <xsl:attribute name="href"><xsl:value-of select="@url"/></xsl:attribute>
      <xsl:apply-templates/>
   </xsl:element>
</xsl:template>

This sucessfully dealt with the first form of <ulink> that had content, but not with the second example with an empty element. The solution is the use an <xsl:choose> element with a test to see if the current node has any child nodes. Using child::node() we can get any child nodes. We can then test if the node has any children using the count() function. The resulting xslt is:

<xsl:template match="ulink">
   <xsl:element name="a">
      <xsl:attribute name="href"><xsl:value-of select="@url"/></xsl:attribute>
      <xsl:choose>
         <xsl:when test="count(child::node())">
            <xsl:apply-templates/>
         </xsl:when>
         <xsl:otherwise>
            <xsl:value-of select="@url"/>
         </xsl:otherwise>
      </xsl:choose>
   </xsl:element>
</xsl:template>