current.value = value;
    }
  }

  /** An {@code Iterator} over distinct keys in key head order. */
  private class DistinctKeyIterator implements Iterator<K> {
    final Set<K> seenKeys = Sets.<K>newHashSetWithExpectedSize(keySet().size());
    @CheckForNull Node<K, V> next = head;
    @CheckForNull Node<K, V> current;
    int expectedModCount = modCount;

    private void checkForConcurrentModification() {