Incrementing a variable in XSL

Hello ,
I have a simple XML file as below:
<?xml version="1.0" encoding="UTF-8"?>
<docroot>
  <a>1</a>
  <a>2</a>
  <a>3</a>
  <a>4</a>
</docroot>

I have written a XSL stylesheet as below:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:template match="docroot">
    <xsl:variable name="var">1</xsl:variable>
    <xsl:for-each select="a">
       <xsl:value-of select="$var"/>  
       <!-- incrementing $var ?? --> 
    </xsl:for-each>    
</xsl:template>
</xsl:stylesheet>

I want to increment a variable in <xsl:for-each ..,
and print it with <xsl:value-of ..

There is a situation where in the XML file, a tag
repeats and I want to output the no's 1, 2, 3 etc in
the output with each tag instance.

How can I do this?

Regards,
Mukul



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