anchor.diff:        options = mkOption {
           default = "";
           example = "--max-freed $((64 * 1024**3))";
  +        type = types.string;
           description = "
             Options given to <filename>nix-collect-garbage</filename> when the
             garbage collector is run automatically.
  
positive.code: 
        automatic = mkOption {
          default = false;
          example = true;
          description = "
            Automatically run the garbage collector at specified dates.
          ";
  
negative.code: 
    ###### implementation
  
    config = {
  
      services.cron.systemCronJobs = mkIf cfg.automatic (singleton
        "${cfg.dates} root ${config.system.build.systemd}/bin/systemctl start nix-gc.service");
  
      boot.systemd.services."nix-gc" =
        { description = "Nix Garbage Collector";
          serviceConfig.ExecStart =
            "@${nix}/bin/nix-collect-garbage nix-collect-garbage ${cfg.options}";
        };
  
    };
  
  }
  