* @param o the object to remove from {@code c}
   * @return {@code true} if {@code c} was changed
   */
  static <K extends @Nullable Object, V extends @Nullable Object> boolean removeEntryImpl(
      Collection<Entry<K, V>> c, @Nullable Object o) {
    if (!(o instanceof Entry)) {
      return false;
    }
    return c.remove(unmodifiableEntry((Entry<?, ?>) o));
  }