anchor.diff:      # unfortunately no version number
       url = "https://asciidoc-diag-filter.googlecode.com/files/diag_filter.zip";
       sha256 = "1qlqrdbqkdqqgfdhjsgdws1al0sacsyq6jmwxdfy7r8k7bv7n7mm";
  +  };
  +
  +  qrcodeFilterSrc = fetchurl {
  +    url = "https://asciidoc-qrencode-filter.googlecode.com/files/qrcode-filter-1.0.zip";
  +    sha256 = "0h4bql1nb4y4fmg2yvlpfjhvy22ln8jsaxdr10f8bfcg5lr0zkxs";
     };
   
   in
  
positive.code:         -e "s|filter='nwdiag|filter=\'${nwdiag}/bin/nwdiag|" \
          -e "s|filter='packetdiag|filter=\'${nwdiag}/bin/packetdiag|" \
          "$out/etc/asciidoc/filters/diag/diag-filter.conf"
    '' + ''
      for n in $(find "$out" . -name \*.py); do
        sed -i -e "s,^#![[:space:]]*/usr/bin/env python,#!${python}/bin/python,g" "$n"
  
negative.code: , enableDitaaFilter ? false, jre ? null
  , enableMscgenFilter ? false, mscgen ? null
  , enableDiagFilter ? false, blockdiag ? null, seqdiag ? null, actdiag ? null, nwdiag ? null
  , enableQrcodeFilter ? false, qrencode ? null
  }:
  
  assert (enableDitaaFilter || enableMscgenFilter || enableDiagFilter || enableQrcodeFilter) -> unzip != null;
  assert enableDitaaFilter -> jre != null;
  assert enableMscgenFilter -> mscgen != null;
  assert enableDiagFilter -> blockdiag != null && seqdiag != null && actdiag != null && nwdiag != null;
  