anchor.diff:  , isPy3k
   , fetchPypi
   , substituteAll
  -, ffmpeg
  +, ffmpeg_4
   , python
   }:
   
  
positive.code: 
      ${python.interpreter} << EOF
      from imageio_ffmpeg import get_ffmpeg_version
      assert get_ffmpeg_version() == '${ffmpeg.version}'
      EOF
  
      runHook postCheck
  
negative.code:   patches = [
      (substituteAll {
        src = ./ffmpeg-path.patch;
        ffmpeg = "${ffmpeg_4}/bin/ffmpeg";
      })
    ];
  
  