anchor.diff:  with {
     inherit (stdenv.lib)
       enFlag
  +    optionals
       optionalString
       wtFlag;
   };
  +
  +assert xorg != null ->
  +  xorg.libX11 != null
  +  && xorg.libXft != null
  +  && xorg.libXrender != null;
   
   stdenv.mkDerivation rec {
     name = "pango-${version}";
  
positive.code:     glib
      gobject-introspection
      harfbuzz
      xorg.libX11
      xorg.libXft
      xorg.libXrender
  
negative.code:     "--disable-doc-cross-reference"
      "--enable-Bsymbolic"
      "--disable-installed-tests"
      (wtFlag "xft" (xorg != null) null)
      (wtFlag "cairo" (cairo != null) null)
    ];
  
  