code: public Map<String, String> getExtParams() {
          if (null == extParams) {
              extParams = new LinkedHashMap<String, String>();
          }
          return extParams;
      }
docs: a map of any extension parameters such as the X-FOO=BAR in RRULE;X-FOO=BAR.
  Maps the parameter name, X-FOO, to the parameter value, BAR.