How to default a variable in XSLT

Since I couldn't find this anywhere on the web, and I'm working on a project that has had me very quickly learn XSLT, here's how to default a value in XSLT - useful if you're looking to grab a variable via the query string, and it may not be there in the first place.

<xsl:variable name="show_comments">
  <xsl:choose>
    <xsl:when test="//QUERY_STRING/show_comments"><xsl:value-of select="//QUERY_STRING/show_comments"/></xsl:when>
    <xsl:otherwise>0</xsl:otherwise> <!-- default value -->
  </xsl:choose>
</xsl:variable>

Note: //QUERY_STRING is a made up variable

I know XSLT might be a bit random for me, but client wants: client gets :-)