anchor.diff:  let
     inherit (lib) concatStrings foldl foldl' genAttrs literalExample maintainers
                   mapAttrsToList mkDefault mkEnableOption mkIf mkMerge mkOption
  -                optional types;
  +                optional types mkOptionDefault flip attrNames;
   
     cfg = config.services.prometheus.exporters;
   
  
positive.code: 
    mkSubModule = { name, port, extraOpts, imports }: {
      ${name} = mkOption {
        type = types.submodule {
          inherit imports;
          options = (mkExporterOpts {
            inherit name port;
          } // extraOpts);
        };
        internal = true;
        default = {};
      };
  
negative.code:       '';
      };
      firewallFilter = mkOption {
        type = types.nullOr types.str;
        default = null;
        example = literalExample ''
          "-i eth0 -p tcp -m tcp --dport ${toString port}"
        '';
  