code: 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);
      }
docs: Appends a {@link ConfigIncluder} which customizes how
  includes are handled.  To append, the library calls {@link
  ConfigIncluder#withFallback} on the existing includer.
  
  @param includer the includer to append (may not be null)
  @return new version of the parse options with different includer