anchor.diff:  
     postUnpack = ''
       mkdir -p $sourceRoot/src/vendor/yices/v2.6/yices2
  -    # XXX: only works because yices.src isn't a tarball.
  -    cp -av ${yices.src}/* $sourceRoot/src/vendor/yices/v2.6/yices2
  +    tar -C $sourceRoot/src/vendor/yices/v2.6/yices2 -xf ${yices-src}
       chmod -R +rwX $sourceRoot/src/vendor/yices/v2.6/yices2
     '';
   
  
positive.code:     libX11 # tcltk
      tcl
      tk
      xorg.libXft
      zlib
    ];
  
negative.code: { lib
  , stdenv
  , fetchFromGitHub
  , fetchurl
  , autoconf
  , automake
  , fontconfig
  