*/
  static <K extends @Nullable Object, V extends @Nullable Object>
      Set<Entry<K, V>> unmodifiableEntrySet(Set<Entry<K, V>> entrySet) {
    return new UnmodifiableEntrySet<>(Collections.unmodifiableSet(entrySet));
  }

  /**
   * Returns an unmodifiable view of the specified map entry. The {@link Entry#setValue} operation