anchor.diff:  
   let
   
  +  cfg = config.networking.wireless;
     configFile = "/etc/wpa_supplicant.conf";
   
     ifaces =
  -    config.networking.wireless.interfaces ++
  +    cfg.interfaces ++
       optional (config.networking.WLANInterface != "") config.networking.WLANInterface;
   
   in
  
positive.code: 
          path = [ pkgs.wpa_supplicant ];
  
          preStart =
            ''
              touch -a ${configFile}
              chmod 600 ${configFile}
            '';
  
          script =
            ''
  
negative.code:         ${config.system.build.upstart}/sbin/restart wpa_supplicant
        '';
  
      assertions = [{ assertion = !cfg.userControlled.enable || cfg.interfaces != []; 
                      message = "user controlled wpa_supplicant needs explicit networking.wireless.interfaces";}];
  
  
    };
  
  }
  