anchor.diff:    postFixup = let
       rpath = stdenv.lib.makeLibraryPath
         (if cudaSupport then
  -        [ gcc49.cc.lib zlib cudatoolkit cudnn
  +        [ stdenv.cc.cc.lib zlib cudatoolkit cudnn
             linuxPackages.nvidia_x11 ]
         else
  -        [ gcc.cc.lib zlib ]
  +        [ stdenv.cc.cc.lib zlib ]
         );
     in
     ''
  
positive.code: , swig
  , werkzeug
  , mock
  , gcc
  , zlib
  }:
  
  assert cudaSupport -> cudatoolkit != null
                     && cudnn != null
                     && gcc49 != null
                     && linuxPackages != null;
  
  # unsupported combination
  
negative.code: , cudaSupport ? false
  , cudatoolkit ? null
  , cudnn ? null
  , linuxPackages ? null
  , numpy
  , six
  