anchor.diff:  
     propagatedBuildInputs = [
       imageio
  +    imagecorruptions
       numpy
       opencv3
       scikitimage
  
positive.code: { buildPythonPackage
  , fetchFromGitHub
  , imageio
  , numpy
  , opencv3
  , pytest
  
negative.code:     six
    ];
  
    checkPhase = ''
       pytest ./test
    '';
  
    checkInputs = [ opencv3 pytest ];
  