anchor.diff:  
     src = fetchurl {
       url = "${webpage}/releases/${pname}-${version}.tbz";
  -    sha256 = "1cdcvsr5z8845ndilnrz7p4n6yn4gv2p91z2mgi4vrailcmn5vzd";
  +    sha256 = "1czd2fq85hy24w5pllarsq4pvbx9rda5zdikxfxdng8s9kff2h3f";
     };
   
  -  buildInputs = [ ocaml findlib opam ];
  +  buildInputs = [ ocaml findlib opam topkg ];
   
  -  propagatedBuildInputs = [ gg ]
  +  propagatedBuildInputs = [ uchar result gg ]
                             ++ optionals pdfBackend [ uutf otfm ]
                             ++ optionals htmlcBackend [ js_of_ocaml ];
   
  
positive.code: 
    unpackCmd = "tar xjf $src";
  
    buildPhase = "ocaml pkg/build.ml native=true native-dynlink=true"
                 + (if pdfBackend then " uutf=true otfm=true"
                                  else " uutf=false otfm=false")
                 + (if htmlcBackend then " jsoo=true"
                                    else " jsoo=false");
  
    installPhase = ''
      opam-installer --script --prefix=$out ${pname}.install | sh
      ln -s $out/lib/${pname} $out/lib/ocaml/${getVersion ocaml}/site-lib/${pname}
    '';
  
    meta = with stdenv.lib; {
      description = "Declarative 2D vector graphics for OCaml";
  
negative.code: { stdenv, fetchurl, ocaml, findlib, opam, topkg
  , uchar, result, gg, uutf, otfm, js_of_ocaml,
    pdfBackend ? true, # depends on uutf and otfm
    htmlcBackend ? true # depends on js_of_ocaml
  }:
  
  let
    inherit (stdenv.lib) optionals versionAtLeast;
  
    pname = "vg";
    version = "0.9.0";
    webpage = "http://erratique.ch/software/${pname}";
    sob = b: if b then "true" else "false";
  in
  
  assert versionAtLeast ocaml.version "4.02.0";
  
  stdenv.mkDerivation rec {
  
  