anchor.diff:  
     pika = buildPythonPackage {
       name = "pika-0.9.12";
  +    diabled = isPy3k;
       src = fetchurl {
         url = https://pypi.python.org/packages/source/p/pika/pika-0.9.12.tar.gz;
         md5 = "7174fc7cc5570314fa3cfaa729106482";
  
positive.code: 
    paste = buildPythonPackage rec {
      name = "paste-1.7.5.1";
  
      src = fetchurl {
        url = http://pypi.python.org/packages/source/P/Paste/Paste-1.7.5.1.tar.gz;
  
negative.code: 
    mechanize = buildPythonPackage (rec {
      name = "mechanize-0.1.11";
      disabled = isPy3k;
  
      src = fetchurl {
        url = "http://wwwsearch.sourceforge.net/mechanize/src/${name}.tar.gz";
  