anchor.diff:    }
   
     _NodeBase<K, V>
  -      _unionWith(_Owner owner, _ANodeBase<K, V> m, V combine(V x, V y), int depth) =>
  +      _unionWith(_Owner owner, _NodeBase<K, V> m, V combine(V x, V y), int depth) =>
             m._unionWithLeaf(owner, this, combine, depth);
   
     _NodeBase<K, V>
  
positive.code:   }
  
    _NodeBase mapValues(_Owner owner, f(V)) {
      List<_ANodeBase<K, V>> newarray = _makeCopyIfNeeded(owner, this._owner, _array);
      for (int i = 0; i < _array.length; i++) {
        _ANodeBase<K, V> mi = _array[i];
          newarray[i] = mi.mapValues(owner, f);
      }
      return new _SubMap.ensureOwner(this, owner, _bitmap, newarray, length);
  
negative.code:     return new _SubMap<K, V>.ensureOwner(this, owner, ormap, newarray, newSize);
    }
  
    _NodeBase<K, V> _intersectionWith(_Owner owner, _NodeBase<K, V> m,
                                          V combine(V x, V y), int depth) =>
        m._intersectionWithSubMap(owner, this, combine, depth);
  
  