if (keyList == null) {
        keyToKeyList.put(key, keyList = new KeyList<>(node));
        modCount++;
      } else {
        keyList.count++;
        Node<K, V> keyTail = keyList.tail;
        keyTail.nextSibling = node;
        node.previousSibling = keyTail;
        keyList.tail = node;
      }
    } else { // non-empty list, insert before nextSibling
      /*