docs: public ConfigParseOptions appendIncluder(ConfigIncluder includer) {
          if (includer == null)
              throw new NullPointerException("null includer passed to appendIncluder");
          if (this.includer == includer)
              return this;
          else if (this.includer != null)
              return setIncluder(this.includer.withFallback(includer));
          else
              return setIncluder(includer);
      }