Building Dynamic Urls

Hi all,

  I am wanting to build dynamic urls in xslt by using string concatenation,
unless there is a better way. My stylesheet will be sent a $url parameter to
provide the base of the url. From their my logic is as follows:
    
<xsl:if test="child::area">
  Building_url = $url and "?area=" and <xsl:value-of select="area"/>
</xsl:if>

<xsl:if test="child::action">
  Building_url = $url and "&action=" and <xsl:value-of select="action"/>
</xsl:if>

<xsl:if test="child::page">
  Building_url = $url and "&page=" and <xsl:value-of select="page"/>
</xsl:if>

I as I understand it, xsl variables can't be changed once given a value.
So what would be the best way to go about building these dynamic urls.

Help appreciated!

Cheers, 
Adam