anchor.diff:      '');
   
     buildInputs =
  -       [ zlib pcre hdf5 protobuf ]
  +       [ zlib pcre hdf5 ]
       ++ lib.optional enablePython pythonPackages.python
       ++ lib.optional enableGtk2 gtk2
       ++ lib.optional enableGtk3 gtk3
  
positive.code:     src = fetchFromGitHub {
        owner  = "opencv";
        repo   = "opencv_3rdparty";
        rev    = "a62e20676a60ee0ad6581e217fe7e4bada3b95db";
        sha256 = "04idycc479l7fidj6r107sv65iszndswm287ms3nh896jbpbaxbv";
      } + "/ippicv";
      files = let name = platform : "ippicv_2017u2_${platform}_20170418.tgz"; in
        if stdenv.system == "x86_64-linux" then
        { ${name "lnx_intel64"} = "87cbdeb627415d8e4bc811156289fa3a"; }
        else if stdenv.system == "i686-linux" then
        { ${name "lnx_ia32"}    = "f2cece00d802d4dea86df52ed095257e"; }
        else if stdenv.system == "x86_64-darwin" then
        { ${name "mac_intel64"} = "0c25953c99dbb499ff502485a9356d8d"; }
        else
        throw "ICV is not available for this platform (or not yet supported by this package)";
      dst = ".cache/ippicv";
  
negative.code:     dst = ".cache/xfeatures2d/boostdesc";
    };
  
    # See opencv/cmake/OpenCVDownload.cmake
    installExtraFiles = extra : with lib; ''
      mkdir -p "${extra.dst}"
    '' + concatStrings (mapAttrsToList (name : md5 : ''
  