sclass WeakIdentityHashSet<A> extends AbstractSet<A> {
  Map<A, Bool> map = newWeakIdentityHashMap();
  
  public int size() { ret map.size(); }
  public Iterator<A> iterator() { ret keys(map).iterator(); }
  public bool contains(O o) { ret map.containsKey(o); }
  
  public bool add(A a) {
    ret map.put(a, Bool.TRUE) != null;
  }
  public bool remove(O o) { ret map.remove(o) != null; }
  
  O mutex() { ret collectionMutex(map); }
}

Began life as a copy of #1011596