anchor.diff:  
           # Test nix-env.
           $machine->mustFail("hello");
  -        $machine->mustSucceed("nix-env -i hello");
  +        $machine->mustSucceed("nix-env -f /etc/nixos/nixpkgs -i hello");
           $machine->mustSucceed("hello") =~ /Hello, world/
               or die "bad `hello' output";
         ''}
  
positive.code:     { services.httpd.enable = true;
        services.httpd.adminAddr = "foo@example.org";
        services.httpd.servedDirs = singleton
          { urlPath = "/releases/nixpkgs/channels/nixpkgs-unstable";
            dir = "/tmp/channel";
          };
  
  
negative.code:         $webserver->mustSucceed("mkdir /tmp/channel");
          $webserver->mustSucceed(
              "nix-push file:///tmp/channel " .
              "http://nixos.org/releases/nixos/channels/nixos-unstable " .
              "file:///tmp/channel/MANIFEST ${toString channelContents} >&2");
        ''}
  
  