anchor.diff:          return this;
         }
         List<_APersistentMap<K, V>> newarray =
  -          new List<_APersistentMap<K, V>>.from(_array);
  +          new List<_APersistentMap<K, V>>.from(_array, growable: false);
         newarray[index] = newm;
         return new _SubMap(_bitmap, newarray, length);
       } else {
  
positive.code: 
      if ((_bitmap & mask) != 0) {
        List<_APersistentMap<K, V>> newarray =
            new List<_APersistentMap<K, V>>.from(_array);
        _APersistentMap<K, V> m = _array[index];
        _APersistentMap<K, V> newM =
            m._insertWith(keyValues, size, combine, hash, depth + 1);
  
negative.code:           return newm;
          } else {
            List<_APersistentMap<K, V>> newarray =
                new List<_APersistentMap<K, V>>.from(_array, growable: false);
            newarray[index] = newm;
            return new _SubMap(_bitmap, newarray, length + delta);
          }
        } else {
          List<_APersistentMap<K, V>> newarray =
              new List<_APersistentMap<K, V>>.from(_array, growable: false);
          newarray[index] = newm;
          return new _SubMap(_bitmap, newarray, length + delta);
        }
  