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