@Override public Iterator<Entry<K, V>> iterator() {
      return new LinkedTreeMapIterator<Entry<K, V>>() {
        public Entry<K, V> next() {
          return nextNode();
        }
      };
    }
This stack is a singly linked list, linked by the 'parent' field.
lottie/src/main/java/com/airbnb/lottie/parser/moshi/LinkedHashTreeMap.java