* element, or at the end of the list if {@code nextSibling} is null. Note: if {@code nextSibling}
   * is specified, it MUST be for a node for the same {@code key}!
   */
  @CanIgnoreReturnValue
  private Node<K, V> addNode(
      @ParametricNullness K key, @ParametricNullness V value, @Nullable Node<K, V> nextSibling) {
    Node<K, V> node = new Node<>(key, value);
    if (head == null) { // empty list
      head = tail = node;