How to retrieve and use chapter numbers in AntennaHouse PDF generation with DITA Open Toolkit?

Question

I am a novice in stylesheet development, and my approach is still very trial & error. At the moment I am trying to retrieve the number of the chapter (there's already a template in the stylesheet I am working on) to be able to use it in a when statement

<xsl:template match="*" mode="randomName">
  <fo:marker marker-class-name="current-chapter-number">
      ...
      ...
  </fo:marker>
</xsl:template>

<xsl:template name="randomName2">
  <xsl:param name="flow-part" select="'body'"/>
  <xsl:variable name="nr">
    <fo:retrieve-marker retrieve-class-name="current-chapter-number"/>
  </xsl:variable-->
  <fo:static-content flow-name="odd-{$flow-part}-tabs">
    <fo:block-container>
      <fo:block>      
        <xsl:value-of select="$nr"/>
      </fo:block>   
    </fo:block-container>
  </fo:static-content>
</xsl:template>

I tried in many different ways but I am probably ignoring the marker functioning and its correct use. Any help or pointers to good resources is much appreciated, thank you!