code: public void pushUlterior(String key, Object value) {
          assertArgumentNotNull("key", key);
          assertArgumentNotNull("value", value);
          if (pushedUlteriorMap == null) {
              pushedUlteriorMap = new LinkedHashMap<String, Object>(4);
          }
          pushedUlteriorMap.put(key, value);
      }
docs: basically unused in mailflute, this is for extension by application