anchor.diff:          -e "s|^ASCIIDOC =.*|ASCIIDOC = '$out/bin/asciidoc'|" \
           -e "s|^XSLTPROC =.*|XSLTPROC = '${libxslt.bin}/bin/xsltproc'|" \
           -e "s|^DBLATEX =.*|DBLATEX = '${dblatexFull}/bin/dblatex'|" \
  -        -e "s|^FOP =.*|FOP = '${fop}/bin/fop'|" \
  +        ${optionalString enableJava ''-e "s|^FOP =.*|FOP = '${fop}/bin/fop'|"''} \
           -e "s|^W3M =.*|W3M = '${w3m}/bin/w3m'|" \
           -e "s|^LYNX =.*|LYNX = '${lynx}/bin/lynx'|" \
           -e "s|^XMLLINT =.*|XMLLINT = '${libxml2.bin}/bin/xmllint'|" \
  
positive.code: 
  # filters
  assert enableExtraPlugins || enableDitaaFilter || enableMscgenFilter || enableDiagFilter || enableQrcodeFilter || enableAafigureFilter -> unzip != null;
  assert enableExtraPlugins || enableDitaaFilter -> jre != null;
  assert enableExtraPlugins || enableMscgenFilter -> mscgen != null;
  assert enableExtraPlugins || enableDiagFilter -> blockdiag != null && seqdiag != null && actdiag != null && nwdiag != null;
  assert enableExtraPlugins || enableMatplotlibFilter -> matplotlib != null && numpy != null;
  
negative.code:   docbook_xml_dtd_45 != null &&
    docbook5_xsl != null &&
    docbook_xsl != null &&
    (fop != null || !enableJava) &&
  # TODO: Package this:
  #  epubcheck != null &&
    gnused != null &&
  