anchor.diff:      ar p "$src" data.tar.xz | tar xJ
     '';
   
  -  buildPhase = ":";
  +  dontBuild = true;
   
     nativeBuildInputs = [ makeWrapper ];
   
  
positive.code:   # build dependencies
    alsaLib, atk, cairo, cups, dbus, expat, fontconfig,
    freetype, gdk_pixbuf, glib, gnome2, nspr, nss, xorg,
    glibc, udev
  }:
  
  stdenv.mkDerivation rec {
  
negative.code:     chmod a+x $out/opt/Pencil/libffmpeg.so
  
      # fix missing libudev
      ln -s ${systemd.lib}/lib/libudev.so.1 $out/opt/Pencil/libudev.so.1
      wrapProgram $out/opt/Pencil/pencil \
        --prefix LD_LIBRARY_PATH : $out/opt/Pencil
    '';
  