anchor.diff:      ++ lib.optional enableFfmpeg ffmpeg
       ++ lib.optionals enableGStreamer (with gst_all_1; [ gstreamer gst-plugins-base ])
       ++ lib.optional enableEigen eigen
  +    ++ lib.optional enableCuda [ cudatoolkit gcc49 ]
       ;
   
     propagatedBuildInputs = lib.optional enablePython pythonPackages.numpy;
  
positive.code:     (opencvFlag "JPEG" enableJPEG)
      (opencvFlag "PNG" enablePNG)
      (opencvFlag "OPENEXR" enableEXR)
    ] ++ lib.optionals enableContrib [ "-DOPENCV_EXTRA_MODULES_PATH=${contribSrc}/modules" ];
  
    enableParallelBuilding = true;
  
  
negative.code: , enableFfmpeg ? false, ffmpeg
  , enableGStreamer ? false, gst_all_1
  , enableEigen ? false, eigen
  , enableCuda ? false, cudatoolkit, gcc49
  }:
  
  let
  