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