XSL Global Variable

Hi all,
  Suppose I have defined a variable at the ROOT
  of the XSL document , how can I access that 
  variable and change its value?

<xsl:stylesheet ....
  <xsl:variable name="gvar">0</xsl:variable>

  <xsl:template match="Increment">
  <!--Here i want to do $gvar=$gvar+1 -->
  </xsl:template>

</<xsl:stylesheet>

regards
chandra


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