{ pkgs, options
# revision can have multiple values: local, HEAD or any revision number.
, revision ? "HEAD"
}:

let
  # To prevent infinite recursion, remove system.path from the
  # options.  Not sure why this happens.
  options_ =
    options //
    { system = removeAttrs options.system ["path"]; };

  optionsXML_ = builtins.toFile "options.xml" (builtins.unsafeDiscardStringContext
    (builtins.toXML (pkgs.lib.optionAttrSetToDocList "" options_)));

  optionsXML = pkgs.runCommand "options2.xml" {} ''
    sed '
      \,<attr name="\(declarations\|definitions\)">, {
        n # fetch the next line
        : rewriteLinks
        n # fetch the next line
        \,</list>, b # leave if this is the end of the list
        ${if revision == "local" then "" else ''
        # redirect nixos internals to the repository
        s,<string value="[^"]*/modules/\([^"]*\)" />,<string value="!https://svn.nixos.org/viewvc/nix/nixos/trunk/modules/\1?revision=${revision}!!../nixos/modules/\1!" />, #"
        t rewriteLinks # jump to rewriteLinks if done
        ''}
        # redirect local file to their locations
        s,<string value="\([^"]*\)" />,<string value="!file://\1\!!../nixos/modules/\1!" />, #"
        b rewriteLinks # jump to rewriteLinks
      }
    ' ${optionsXML_} > $out
  '';

  optionsDocBook = pkgs.runCommand "options-db.xml" {} ''
    ${pkgs.libxslt}/bin/xsltproc -o $out ${./options-to-docbook.xsl} ${optionsXML}
  '';

  manual = pkgs.stdenv.mkDerivation {
    name = "nixos-manual";

    sources = pkgs.lib.sourceFilesBySuffices ./. [".xml"];

    buildInputs = [pkgs.libxml2New pkgs.libxslt];

    xsltFlags = ''
      --param section.autolabel 1
      --param section.label.includes.component.label 1
      --param html.stylesheet 'style.css'
      --param xref.with.number.and.title 1
      --param toc.section.depth 3
      --param admon.style '''
      --param callout.graphics.extension '.gif'
    '';

    buildCommand = ''
      
      ln -s $sources/*.xml . # */
      ln -s ${optionsDocBook} options-db.xml

      # Check the validity of the manual sources.
      xmllint --noout --nonet --xinclude --noxincludenode \
        --relaxng ${pkgs.docbook5}/xml/rng/docbook/docbook.rng \
        manual.xml

      # Generate the HTML manual.
      dst=$out/share/doc/nixos
      ensureDir $dst
      xsltproc $xsltFlags --nonet --xinclude \
        --output ./manual.html \
        ${pkgs.docbook5_xsl}/xml/xsl/docbook/xhtml/docbook.xsl \
        ./manual.xml

      sed '
        s,!\([^!]*\)!!\([^!]*\)!,<a class="link" href="\1" target="_top"><code class="filename">\2</code></a>,g
      ' ./manual.html > $dst/manual.html

      ln -s ${pkgs.docbook5_xsl}/xml/xsl/docbook/images $dst/
      cp ${./style.css} $dst/style.css

      # Generate manpages.
      ensureDir $out/share/man
      xsltproc --nonet --xinclude \
        --param man.output.in.separate.dir 1 \
        --param man.output.base.dir "'$out/share/man/'" \
        ${pkgs.docbook5_xsl}/xml/xsl/docbook/manpages/docbook.xsl \
        ./man-pages.xml
      
      ensureDir $out/nix-support
      echo "doc manual $dst manual.html" >> $out/nix-support/hydra-build-products
    '';
  };

in manual