anchor.diff:  
           echo -n "$systemConfig" > $out/systemConfig
   
  -        echo -n "$nixdarwinLabel" > $out/nixdarwin-version
  +        echo -n "$darwinLabel" > $out/darwin-version
           echo -n "$system" > $out/system
         '';
       };
  
positive.code:       '';
      };
  
      system.nixdarwinLabel = mkOption {
        type = types.str;
        default = "16.09";
      };
  
    };
  
negative.code:   config = {
  
      system.build.toplevel = stdenvNoCC.mkDerivation {
        name = "darwin-system-${cfg.darwinLabel}";
        preferLocalBuild = true;
  
        activationScript = cfg.activationScripts.script.text;
        activationUserScript = cfg.activationScripts.userScript.text;
        inherit (cfg) darwinLabel;
  
        buildCommand = ''
          mkdir $out
  