anchor.diff:      }
     }
   
  +  Node<K, V> freeze();
  +
     Node<K, V> add(int shift, int hash, K key, V value, Box addedLeaf);
   
     Node<K, V> addM(int shift, int hash, K key, V value, Box addedLeaf);
  
positive.code:   }
  
    @override
    Node<K, V> add(int shift, int hash, K key, V value, Box addedLeaf) {
      int idx = mask(hash, shift);
      Node<K, V> node = list[idx];
  
negative.code:     return _leafFlag.value as V;
    }
  
    ImmutableMap<K, V> toImmutable() => new HamtImmutableMap._internal(
        _length, _root?.freeze(), _hasNull, _nullValue);
  }
  
  typedef SelectorFunction(key, value);
  