anchor.diff:  
     trac = buildPythonPackage {
       name = "trac-0.11.5";
  -    
  +
       src = fetchurl {
         url = http://ftp.edgewall.com/pub/trac/Trac-0.11.5.tar.gz;
         sha256 = "cc3362ecc533abc1755dd78e2d096d1413bc975abc3185318f4821458cd6a8ac";
       };
  -    
  -    doCheck = false;
  -    
  +
  +    doCheck = false;
  +
       PYTHON_EGG_CACHE = "`pwd`/.egg-cache";
  -    
  +
       propagatedBuildInputs = [ genshi pkgs.setuptools ];
  -    
  +
       meta = {
         description = "Enhanced wiki and issue tracking system for software development projects";
  -      
  +
         license = "BSD";
       };
     };
  
positive.code: 
      buildInputs = [ pkgs.libxml2 pkgs.libxslt ];
  
      meta = { 
        description = "Pythonic binding for the libxml2 and libxslt libraries";
        homepage = http://codespeak.net/lxml/index.html;
        license = "BSD";
  
negative.code: 
    genshi = buildPythonPackage {
      name = "genshi-0.5.1";
  
      src = fetchurl {
        url = http://ftp.edgewall.com/pub/genshi/Genshi-0.5.1.tar.bz2;
        sha256 = "1g2xw3zvgz59ilv7mrdlnvfl6ph8lwflwd4jr6zwrca2zhj7d8rs";
      };
  
      buildInputs = [ pkgs.setuptools ];
  
      meta = {
        description = "Python components for parsing HTML, XML and other textual content";
  
        longDescription = ''
          Python library that provides an integrated set of
          components for parsing, generating, and processing HTML, XML or other
          textual content for output generation on the web.
        '';
  
        license = "BSD";
      };
    };
  