anchor.diff:  
     postInstall = ''
       wrapProgram "$out/sbin/bumblebeed" \
  -      --prefix PATH : "${bbdPath}" \
  -      --prefix LD_LIBRARY_PATH : "${bbdLibs}"
  +      --prefix PATH : "${bbdPath}"
   
       wrapProgram "$out/bin/optirun" \
         --prefix PATH : "${virtualgl}/bin"
  
positive.code:   nvidiaLibs = lib.makeLibraryPath nvidia_x11s;
  
    bbdPath = lib.makeBinPath [ kmod xorgserver ];
    bbdLibs = lib.makeLibraryPath [ libX11 libXext ];
  
    xmodules = lib.concatStringsSep "," (map (x: "${x.out or x}/lib/xorg/modules") ([ xorgserver ] ++ lib.optional (!useNvidia) xf86videonouveau));
  
  
negative.code:   primus = if useNvidia then primusLib else primusLib.override { nvidia_x11 = null; };
    primus_i686 = if useNvidia then primusLib_i686 else primusLib_i686.override { nvidia_x11 = null; };
  
    primusLibs = lib.makeLibraryPath ([ primus ] ++ lib.optional (primusLib_i686 != null) primus_i686);
  
    nvidia_x11s = [ nvidia_x11 ]
                  ++ lib.optional nvidia_x11.useGLVND libglvnd
  