anchor.diff:    });
   
     numpy = buildPythonPackage ( rec {
  -    name = "numpy-1.4.1";
  +    name = "numpy-1.6.1";
   
       src = fetchurl {
         url = "mirror://sourceforge/numpy/${name}.tar.gz";
  -      sha256 = "01lf3nc2lp1qkrqnnar50vb7i6y07d1zs6f9yc3kw4p5fd2vhyrf";
  +      sha256 = "1pawfmf7j7pd3mjzhmmw9hkglc2qdirrkvv29m5nsmpf2b3ip2vq";
       };
   
       # TODO: add ATLAS=${pkgs.atlas}
       installCommand = ''
         export BLAS=${pkgs.blas} LAPACK=${pkgs.liblapack}
         python setup.py build --fcompiler="gnu95"
  -      python setup.py install --root="$out"
  +      python setup.py install --prefix=$out
       '';
       doCheck = false;
   
  
positive.code: 
  
    matplotlib = buildPythonPackage ( rec {
      name = "matplotlib-0.99.1.2";
  
      src = fetchurl {
        url = "http://downloads.sourceforge.net/matplotlib/${name}.tar.gz";
        sha256 = "12lhwgkahck795946hb8wp605c912zq9ds8067ybbifqs56q24b9";
      };
  
      doCheck = false;
  
negative.code:     };
    });
  
    reportlab = 
     let freetype = pkgs.lib.overrideDerivation pkgs.freetype (args: { configureFlags = "--enable-static --enable-shared"; });
     in buildPythonPackage rec {
      name = "reportlab-2.5";
  
      src = fetchurl {
        url = "http://pypi.python.org/packages/source/r/reportlab/${name}.tar.gz";
        md5 = "cdf8b87a6cf1501de1b0a8d341a217d3";
      };
  
      buildInputs = [freetype];
      doCheck = false; 
  
      meta = {
        description = "The ReportLab Toolkit. An Open Source Python library for generating PDFs and graphics.";
        homepage = http://www.reportlab.com/;
      };
    };
  
    rdflib = buildPythonPackage (rec {
      name = "rdflib-3.0.0";
  
  