anchor.diff:  { config, lib, pkgs, ... }:
   
   let
  -  inherit (lib) mkEnableOption mkIf mkOption types;
  +  inherit (lib) mkEnableOption mkIf mkOption optionalString types;
   
     generic = variant:
       let
  
positive.code:               <link xlink:href='http://bird.network.cz/'/>
              '';
            };
          };
        };
  
  
negative.code:         environment.etc."bird/${variant}.conf".source = pkgs.writeTextFile {
            name = "${variant}.conf";
            text = cfg.config;
            checkPhase = optionalString cfg.checkConfig ''
              ${pkg}/bin/${birdBin} -d -p -c $out
            '';
          };
  