code: public static void add(Map<String, Class<? extends ICustomFunction>> moreCustomFunctions) {
          fs.putAll(moreCustomFunctions);
          poifs = loadPoiCustomFunctions(fs);
      }
docs: The best way to add more custom functions. Does everything: adds functions, creates new UDFFinder.
  Use this if common case. Use spesific {@link #fs}.putAll() and {@link #poifs} = {@link #loadPoiCustomFunctions(Map)}
  if more flexibility is needed.