anchor.diff:  
     buildInputs =
          [ zlib ]
  -    ++ lib.optional enablePython python2Packages.python
       ++ lib.optional enableGtk2 gtk2
       ++ lib.optional enableJPEG libjpeg
       ++ lib.optional enablePNG libpng
  
positive.code:     ++ lib.optional enableEigen eigen
      ++ lib.optionals stdenv.isDarwin [ Cocoa QTKit ]
      ;
  
    propagatedBuildInputs = lib.optional enablePython python2Packages.numpy;
  
    nativeBuildInputs = [ cmake pkg-config unzip ];
  
  
negative.code: { lib, stdenv, fetchFromGitHub, cmake, pkg-config, unzip
  , zlib
  , enableGtk2 ? false, gtk2
  , enableJPEG ? true, libjpeg
  , enablePNG ? true, libpng
  