XSLT and Mathematics

I've been studying XSLT lately. After I found out that it can produce factorials, I decided to devise an xsl file that could produce a hailstorm sequence:

<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
 <xsl:output method="text"/>
 <xsl:strip-space elements='*'/>
 <xsl:template name="hailstorm">
  <xsl:param name="n" select="1"/>
  <xsl:variable name="sequence">
   <xsl:if test="$n mod 2 = 0">
    <xsl:call-template name="hailstorm">
     <xsl:with-param name="n" select="$n div 2"/>
    </xsl:call-template>
   </xsl:if>
   <xsl:if test="($n mod 2) * ($n - 1) > 0">
    <xsl:call-template name="hailstorm">
     <xsl:with-param name="n" select="$n * 3 + 1"/>
    </xsl:call-template>
   </xsl:if>
  </xsl:variable>
  <xsl:value-of select="$n"/>
  <xsl:if test="$n > 1">
   <xsl:value-of select="', '"/>
  </xsl:if>
  <xsl:value-of select="$sequence"/>
 </xsl:template>
 <xsl:template match="number">
  <xsl:variable name="x" select="."/>
  <xsl:variable name="y">
   <xsl:call-template name="hailstorm">
    <xsl:with-param name="n" select="$x"/>
   </xsl:call-template>
  </xsl:variable>
  <xsl:value-of select="'hailstorm('"/>
  <xsl:value-of select="$x"/>
  <xsl:value-of select="') = '"/>
  <xsl:value-of select="$y"/>
  <xsl:value-of select="'&#xA;'"/>
 </xsl:template>
</xsl:stylesheet>

You can start by naming the above file "hailstor.xsl" and running it on the following:

<?xml version="1.0"?>
<?xml-stylesheet href="hailstor.xsl" type="text/xsl"?>
<num>27</num>