docs: public PersistentHashArrayMappedTrie<K,V> put(K key, V value) {
      if (root == null) {
        return new PersistentHashArrayMappedTrie<>(new Leaf<>(key, value));
      } else {
        return new PersistentHashArrayMappedTrie<>(root.put(key, value, key.hashCode(), 0));
      }
    }