nixpkgs/system/etc/fonts/make-fonts-conf.xsl

<?xml version="1.0"?>

<!--
  This script copies the original fonts.conf from the fontconfig
  distribution, but replaces all <dir> entries with the directories
  specified in the $fontDirectories parameter.
-->

<xsl:stylesheet version="1.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:str="http://exslt.org/strings"
                extension-element-prefixes="str"
                >

  <xsl:output method='xml' encoding="UTF-8" doctype-system="fonts.dtd" />

  <xsl:param name="fontDirectories" />

  <xsl:template match="/fontconfig">

    <fontconfig>
      <xsl:copy-of select="child::node()[name() != 'dir']" />

      <xsl:for-each select="str:tokenize($fontDirectories)">
        <dir><xsl:value-of select="." /></dir>
        <xsl:text>&#0010;</xsl:text>
      </xsl:for-each>
      
    </fontconfig>

  </xsl:template>
  
</xsl:stylesheet>