sclass WeightedSet<A> extends WrappedMap<A, Double> {
  *() { super(new HashMap); }
  
  bool contains(A a) { ret containsKey(a); }
  public Double get(O a) { ret or0(super.get(a)); }
  double getWeight(A a) { ret get(a); }
  void add(A a) { put(a, 1.0); }
}