public @Nullable Entry<K, Collection<V>> pollFirstEntry() {
      return pollAsMapEntry(entrySet().iterator());
    }

    @Override
    public @Nullable Entry<K, Collection<V>> pollLastEntry() {
      return pollAsMapEntry(descendingMap().entrySet().iterator());
    }

    @Nullable Entry<K, Collection<V>> pollAsMapEntry(
        Iterator<Entry<K, Collection<V>>> entryIterator) {