Loops and variables question

Hi,
I want to build a list with the quantity attribute of the all ROOMS nodes.
I have this XML:

<ROOMS>
 <ROOM quantity="5">
 <ROOM quantity="3">
 <ROOM quantity="7">
</ROOMS>

My desired output is:
1,2,3,4,5,1,2,3,1,2,3,4,5,6,7

I tryed with an auxiliary variable <xsl:variable name="num"...>but I can
increment this variable for each node.
My code is something like this:

<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
	<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
	<xsl:variable name="num" select="1"></xsl:variable>
  <xsl:template match="ROOMS/ROOM">
    <xsl:for-each select="//*[$num&lt;=@quantity]">
    <xsl:value-of select="$num"/><xsl:text>,</xsl:text>
    <xsl:variable name="num" select="number(num) +1"></xsl:variable>
  </xsl:for-each>
 </xsl:template>
</xsl:stylesheet>

How can implement this if a variable cannot be redefined??




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