private final class ValueForKeyIterator implements ListIterator<V> {
    @ParametricNullness final K key;
    int nextIndex;
    @Nullable Node<K, V> next;
    @Nullable Node<K, V> current;
    @Nullable Node<K, V> previous;

    /** Constructs a new iterator over all values for the specified key. */
    ValueForKeyIterator(@ParametricNullness K key) {
      this.key = key;