anchor.diff:                <link xlink:href='http://bird.network.cz/'/>
               '';
             };
  +          checkConfig = mkOption {
  +            type = types.bool;
  +            default = true;
  +            description = ''
  +              Whether the config should be checked at build time.
  +              Disabling this might become necessary if the config includes files not present during build time.
  +            '';
  +          };
           };
         };
   
  
positive.code:         environment.etc."bird/${variant}.conf".source = pkgs.writeTextFile {
            name = "${variant}.conf";
            text = cfg.config;
            checkPhase = ''
              ${pkg}/bin/${birdBin} -d -p -c $out
            '';
          };
  
negative.code: { config, lib, pkgs, ... }:
  
  let
    inherit (lib) mkEnableOption mkIf mkOption optionalString types;
  
    generic = variant:
      let
  