anchor.diff:  , enableEigen     ? false, eigen
   , enableOpenblas  ? false, openblas
   , enableCuda      ? false, cudatoolkit, gcc5
  +, enableTesseract ? false, tesseract, leptonica
   , AVFoundation, Cocoa, QTKit
   }:
   
  
positive.code:   inherit version src;
  
    postUnpack =
      (lib.optionalString enableContrib ''
        cp --no-preserve=mode -r "${contribSrc}/modules" "$NIX_BUILD_TOP/opencv_contrib"
  
        # This fixes the build on OS X.
  
negative.code:     sha256 = "11dsq8dwh1k6f7zglbc26xwsjw184ggf2531mhf7v77kd72k19fm";
    };
  
    # Contrib must be built in order to enable Tesseract support:
    buildContrib = enableContrib || enableTesseract;
  
    vggFiles = fetchFromGitHub {
      owner  = "opencv";
      repo   = "opencv_3rdparty";
  