sclass AppendableChain<A> extends Chain<A> {
  A element;
  Chain<A> next, last;
  int size;

  *() {}
  *(A *element) { size = 1; last = this; }
  *(A *element, Chain<A> *next) {
    if (next != null) {
      size = next.size;
      last = next.last;
    } else
      last = this;
    size++;
  }
  
  toString { ret str(toList()); }
  
  bool add(A a) {
    AppendableChain newLast = new Chain(a, null);
    last.next = newLast;
    last = newLast;
    ++size;
    true;
  }
  
  ArrayList<A> toList() {
    ArrayList<A> l = emptyList(size);
    Chain<A> c = this;
    while (c != null) {
      l.add(c.element);
      c = c.next;
    }
    ret l;
  }
  
  // TODO: optimize
  public Iterator<A> iterator() { ret toList().iterator(); }
}