anchor.diff:      optionals
       optionalString
       platforms;
  -in
  -
  -let
  +
     optionalPlugins = {
       # TODO: write a generic function to detect null dependencies
       acousticbrainz =
  
positive.code: 
    testShell = "${bash}/bin/bash --norc";
    completion = "${bash-completion}/share/bash-completion/bash_completion";
  in
  
  buildPythonPackage rec {
    name = "beets-${version}";
    version = "1.3.19";
    namePrefix = "";
  
    src = fetchFromGitHub {
      owner = "sampsyo";
  
negative.code:       test/test_replaygain.py
    '';
  
    GST_PLUGIN_PATH = makeSearchPath "lib/gstreamer-1.0" [
      gst-plugins-base
      gst-plugins-good
      #gst-plugins-bad
      #gst-plugins-ugly
    ];
  
    preFixup = ''
      wrapProgram $out/bin/beet \
        --prefix 'GI_TYPELIB_PATH' : "$GI_TYPELIB_PATH" \
  