docs: private String getPrefixKey(String key, SortedMap<String, ?> whiteList)
    {
      String prefixKey = null;
      if (whiteList.containsKey(key)) {
        return key;
      }
      SortedMap<String, ?> headMap = whiteList.headMap(key);
      if (!headMap.isEmpty() && key.startsWith(headMap.lastKey())) {
        prefixKey = headMap.lastKey();
      }
      return prefixKey;
    }