anchor.diff:      environment = {
         extraPackages = pkgs: [
           pkgs.vim
  +        pkgs.subversion # for nixos-checkout
         ];
       };
      
  
positive.code:   # Get a recent copy of Nixpkgs.
    nixpkgsTarball = pkgs.fetchurl {
      url = configuration.installer.nixpkgsURL + "/" + nixpkgsRel + ".tar.bz2";
      md5 = "22deefec0696d2fb263864b97e2238d3";
    };
  
  
  
negative.code: rec {
  
    
    nixpkgsRel = "nixpkgs-0.12pre10056";
  
  
    configuration = {
  