anchor.diff:    }
   
     bool operator ==(PersistentMap<K, V> other) {
  -    if (this === other) return true;
  +    if (identical(this, other)) return true;
       if (other is! _SubMap) return false;
       if (_bitmap != other._bitmap) return false;
       if (length != other.length) return false;
  
positive.code:       int index = _popcount(_bitmap & (mask - 1));
        _APersistentMap<K, V> m = _array[index];
        _APersistentMap<K, V> newm = m._adjust(key, update, hash, depth + 1);
        if (newm === m) {
          return this;
        }
        List<_APersistentMap<K, V>> newarray =
  
negative.code:       _APersistentMap<K, V> m = _array[index];
        _APersistentMap<K, V> newm = m._delete(key, hash, depth + 1);
        int delta = newm.length - m.length;
        if (identical(m, newm)) {
          return this;
        }
        if (newm._isEmpty()) {
  