}

  /** An implementation of {@link Map#putAll}. */
  static <K extends @Nullable Object, V extends @Nullable Object> void putAllImpl(
      Map<K, V> self, Map<? extends K, ? extends V> map) {
    for (Entry<? extends K, ? extends V> entry : map.entrySet()) {
      self.put(entry.getKey(), entry.getValue());
    }
  }

  static class KeySet<K extends @Nullable Object, V extends @Nullable Object>