return null;
    }
  }

  /** An admittedly inefficient implementation of {@link Map#containsKey}. */
  static boolean containsKeyImpl(Map<?, ?> map, @CheckForNull Object key) {
    return Iterators.contains(keyIterator(map.entrySet().iterator()), key);
  }

  /** An implementation of {@link Map#containsValue}. */
  static boolean containsValueImpl(Map<?, ?> map, @CheckForNull Object value) {