sclass Chain<A> {
  A element;
  Chain<A> prev;
  int size;
  
  *() {}
  *(Chain<A> *prev, A *element) {
    size = prev != null ? prev.size+1 : 1;
  }
  
  toString {
    ret prev == null ;
  }
  
  L<A> toList() {
    L<A> l = emptyList(size);
    int i = size;
    Chain c = this;
    while (c != null) {
      l.set(--i, c);
      c = c.prev;
    }
    ret l;
  }
}