return Iterators.contains(keyIterator(map.entrySet().iterator()), key);
  }

  /** An implementation of {@link Map#containsValue}. */
  static boolean containsValueImpl(Map<?, ?> map, @CheckForNull Object value) {
    return Iterators.contains(valueIterator(map.entrySet().iterator()), value);
  }

  /**
   * Implements {@code Collection.contains} safely for forwarding collections of map entries. If