gentoo-overlay/app-text/docbook-xsl-stylesheets/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
<maintainer><email>maintainer-needed@gentoo.org</email></maintainer>
  <use>
    <flag name="ruby">
      Install the Ruby-based dbtoepub script; requires an interpreter
      compatible with <pkg>app-admin/eselect-ruby</pkg>.
    </flag>
  </use>
</pkgmetadata>