static Map.Entry[] childrenArray(KeyFMap map) {
    Key[] keys = map.getKeys();
    int length = map.size();
    Map.Entry[] res = new Map.Entry[length];
    for (int i = 0; i < length; i++) {
      Key key = keys[i];
      res[i] = new AbstractMap.SimpleImmutableEntry<>(key, map.get(key));
    }
    return res;
  }
static Map.Entry[] childrenArray(KeyFMap map)