Comparing and re-assigning variables.

Hi all,
I haven't worked a lot with xsl so my question might be easy for you but it's bugging me at the moment. I've tried to find answers in the history but it either goes over my head or it doesn't really answer my question.

Ok, here's a part of a xsl file i wrote to create a transactions report:

<!-- Iterate over all txns. -->
<table border="0">
<xsl:for-each select="report-root/txns">
    <xsl:sort select="cpmBI"/>
    <xsl:sort select="date"/>
    <xsl:variable name="currCpmBI" select="cpmMemBI"/>
    
    <!-- CODE TO BE INSERTED HERE - READ ON -->    

</xsl:for-each>
</table>

What i want is to compare the value of currCpmBI with the previous value in the iteration (which i have not defined in the above code). If they differ I want to add a some extra html.
My question now boils down to:
- How can i remember the previous value of cpmMemBI and how would I store it so i can compare it with the current value in the iteration? 
- If the current value has changed, how can i re-initialise the previous value with the current value?

Hope my question is clear.... ;-)

Cheers,
Bill

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