Number format needed as prefix for the xhtml files of dita ot epub output

I have used the Dita OT 1.8.5 for epub conversion: but i need the number sequence format 01, 02, 03 .. prefix for the xhtml files names in the epub output.

I have changed the code in html-generation-utils.xsl function as below for the number sequence for file names:

 

   

 <xsl:function name="htmlutil:constructHtmlResultTopicFilename" as="xs:string">
        <xsl:param name="topic" as="document-node()"/>
        <xsl:variable name="topicFilename"
          select="concat(htmlutil:getResultTopicBaseName($topic), $OUTEXT)"
          as="xs:string"/>
        <xsl:for-each select="$topic">
    <xsl:variable name="number"><xsl:value-of select="format-number(position(),'00')"/></xsl:variable>
    <xsl:sequence select="concat($number, $topicFilename)"/>
    </xsl:for-each>
      </xsl:function>

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I am getting the output of the file names for all prefix as '01' only.

But i need prefix as 01, 02, 03, ...

Please suggest.

Thanks in Advance