code: @SuppressWarnings("unchecked")
    private HamtPMap<K, V> pivot(K key, int hash) {
      return pivot(key, hash, null, (V[]) new Object[1]);
    }
docs: Returns a new version of this map with the given key at the root, and the root element moved to
  some deeper node. If the key is not found, then value will be null.