anchor.diff:            (machine: ''
               host ${machine.hostName} {
                 hardware ethernet ${machine.ethernetAddress};
  -              fixed-address ${machine.ipAddress};
  +              fixed-address${
  +                optionalString (postfix == "6") postfix
  +              } ${machine.ipAddress};
               }
             '')
             cfg.machines
  
positive.code:   cfg4 = config.services.dhcpd4;
    cfg6 = config.services.dhcpd6;
  
    writeConfig = cfg: pkgs.writeText "dhcpd.conf"
      ''
        default-lease-time 600;
        max-lease-time 7200;
  
negative.code:       configFile =
          if cfg.configFile != null
            then cfg.configFile
            else writeConfig postfix cfg;
        leaseFile = "/var/lib/dhcpd${postfix}/dhcpd.leases";
        args = [
          "@${pkgs.dhcp}/sbin/dhcpd" "dhcpd${postfix}" "-${postfix}"
  