xsl loop with variable

Hi all:
I have the next xsl code:

  <xsl:variable name="numMonth" select="0"/>
  <xsl:variable name="lastMonth" select="00"/>

  <xsl:for-each select="listObject/object">

	<xsl:if test="not($lastMonth=substring(editionDate,4,2))">
	  <xsl:variable name="numMonth" select="$numMonth+1"/>
	</xsl:if>
	
	Num months:<xsl:value-of select="$numMonth"/>

	<xsl:variable name="lastMonth" select="substring(editionDate,4,2)"/>

    </xsl:for-each>

I can't make "numMonth" variable update with the correct increment out of the "if" tag, so that "Num months:" show me the correct value (until now only show '0', the original value).
How can i do that? Can anybody help me please?

Thanks.
_______________________________________________________________
Yahoo! Messenger
Nueva versión: Webcam, voz, y mucho más ¡Gratis! 
Descárgalo ya desde http://messenger.yahoo.es


 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list