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