XSLT to check the function supported to xslt or not ??

 XSLT file :
 ---------------------------------------------------

<?xml version="1.0" encoding="iso-8859-1"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <html>
      <body>
        <xsl:choose>
          <xsl:when test="function-available(sum)">       <!--'sum()' function is checking now-->
            <p>Function is supported.</p>
          </xsl:when>
          <xsl:otherwise>
            <p>Function is not supported.</p>
          </xsl:otherwise>
        </xsl:choose>
      </body>
    </html>
  </xsl:template>
</xsl:stylesheet>

----------------------------------------------------

if o/p is : Function is supported       => means function supported by the xslt