how to change variable values ?

hi,

i got the following problem: i want to output a xml-document with a 
xsl-stylesheet. In this output it should, for example, make a line when 
the week changes. I tried to do this by using variables...

-------------
<xsl:variable name="lastweek">
  <!-- Need an Initial value to compare later with currentweek-->
  <xsl:value-of select="Week"/>  
  <!-- Week is a tag from the xml-document -->
</xsl:variable>

<xsl:for-each select="Data/Block">

  <xsl:variable name="currentweek">
    <xsl:value-of select="Week"/> 
  </xsl:variable>

  <xsl:if test="$currentweek > $lastweek">
    <!-- Do something when week changes and set
         the variable lastweek to this week -->
    <xsl:variable name="lastweek">   <!-- foo ;-) -->
      <xsl:value-of select="$currentweek">
    </xsl:variable>
  </xsl:if>
  
  .
  .
  .
</xsl:for-each>
--------------------------

Obviously, it complains, that the variable lastweek cannot be defined 
twice. Is there a way to "just change" the variable lastweek (and not 
to define it again) ?

Thanks
Gero Nickenig

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