anchor.diff:  , withGUI ? true
     , qtbase ? null
     , qtmultimedia ? null
  +  , wrapQtAppsHook ? null
   }:
   
  -assert withGUI -> qtbase != null && qtmultimedia != null;
  +assert withGUI -> qtbase != null && qtmultimedia != null && wrapQtAppsHook != null;
   
   with stdenv.lib;
   
  
positive.code:     (enableFeature withGUI "qt")
    ];
  
    meta = with stdenv.lib; {
      description = "Cross-platform tools for Matroska";
      homepage    = http://www.bunkus.org/videotools/mkvtoolnix/;
  
negative.code:     expat file xdg_utils boost libebml zlib fmt
      libmatroska libogg libvorbis flac cmark
    ] ++ optional  stdenv.isDarwin libiconv
      ++ optionals withGUI [ qtbase qtmultimedia wrapQtAppsHook ];
  
    preConfigure = "./autogen.sh; patchShebangs .";
    buildPhase   = "drake -j $NIX_BUILD_CORES";
  