How to add a global variable to itself (a=a+b) in xsl
===

I would like to increment my global variable `tmp_balance_virt` in `for-each` loop but in each loop `tmp_balance_virt` (a=**a**+b) is resetting to its initial value, how to make it works? 

    <xsl:variable name="tmp_balance_virt" select="/doc/plot/@st_balance_virt" /> //first value  
    ...
    <xsl:if ...>
    ...
    <xsl:for-each
    	select="/doc/plot/cfc/pl.cashflow.rabean.TransactionsData[type=0]">
    				<xsl:variable name="tmp_balance_virt" select="value + $tmp_balance_virt" />
    				[
    				<xsl:value-of select="date_ms" />
    				,
    				<xsl:value-of select="$tmp_balance_virt" />
    				],
    </xsl:for-each>