sclass UniqueList<A> extends RandomAccessAbstractList<A> {
  new L<A> list;
  new Map<A, Int> index; // position of each element
  
  public A get(int i) { ret list.get(i); }
  public int size() { ret list.size(); }
  
  // couldn't come up with a better name here... add element or
  // get its index if it's already there
  int addOrGetIndex(A a) {
    Int i = index.get(a);
    if (i == null) {
      index.put(a, i = size());
      list.add(a);
    }
    ret i;
  }
  
  public bool add(A a) {
    int n = size();
    ret addOrGetIndex(a) == n;
  }
}