anchor.diff:  
   with stdenv.lib;
   
  -let 
  +let
     available = x: x != null;
   in
   assert x11Support         -> all available [mesa libX11 libXext libXxf86vm];
   assert waylandSupport     -> all available [wayland libxkbcommon];
  +assert rubberbandSupport  -> available rubberband;
   assert xineramaSupport    -> x11Support && available libXinerama;
   assert xvSupport          -> x11Support && available libXv;
   assert sdl2Support        -> available SDL2;
  
positive.code: { stdenv, fetchurl, fetchFromGitHub, makeWrapper
  , docutils, perl, pkgconfig, python3, which, ffmpeg
  , freefont_ttf, freetype, libass, libpthreadstubs
  , lua, lua5_sockets, libuchardet, rubberband
  
  , x11Support ? true,
      mesa       ? null,
  
negative.code:     homepage = http://mpv.io;
      license = licenses.gpl2Plus;
      maintainers = with maintainers; [ AndersonTorres fuuzetsu ];
      platforms = platforms.darwin ++ platforms.linux;
  
      longDescription = ''
        mpv is a free and open-source general-purpose video player,
  