anchor.diff:      environment = {
         extraPackages = pkgs: [
           pkgs.vim
  +        pkgs.subversion # for nixos-checkout
         ];
       };
      
  
positive.code: rec {
  
    
    nixpkgsRel = "nixpkgs-0.12pre9486";
  
  
    configuration = {
  
negative.code:   # Get a recent copy of Nixpkgs.
    nixpkgsTarball = pkgs.fetchurl {
      url = configuration.installer.nixpkgsURL + "/" + nixpkgsRel + ".tar.bz2";
      md5 = "a25cd6eb5e753703823fdd3ae86e9f25";
    };
  
  
  