anchor.diff:  { stdenv, fetchurl, pkgconfig, libpng, libjpeg, expat, libXaw
  -, yacc, libtool, fontconfig, pango, gd, xlibs
  +, yacc, libtool, fontconfig, pango, gd, xlibs, gts
   }:
   
   stdenv.mkDerivation rec {
  
positive.code: 
    buildInputs =
      [ pkgconfig libpng libjpeg expat libXaw yacc libtool fontconfig
        pango gd
      ] ++ stdenv.lib.optionals (xlibs != null) [ xlibs.xlibs xlibs.libXrender ];
    
    configureFlags =
      [ "--with-pngincludedir=${libpng}/include"
        "--with-pnglibdir=${libpng}/lib"
  
negative.code:   '';
  
    meta = {
      homepage = "http://www.graphviz.org/";
      description = "open source graph visualization software"
  
      longDescription = ''
        Graphviz is open source graph visualization software. Graph
        visualization is a way of representing structural information as
        diagrams of abstract graphs and networks. It has important
        applications in networking, bioinformatics, software engineering,
        database and web design, machine learning, and in visual
        interfaces for other technical domains.
      '';
  
      platforms = stdenv.lib.platforms.linux;
      maintainers = [ stdenv.lib.maintainers.simons ];
    };
  }
  