sclass WeakCompactHashSet<A> extends AbstractSet<A> {
  new CompactHashSet<MyWeakReference<A>> set;
  new ReferenceQueue<A> queue;
  
  sclass MyWeakReference<A> extends WeakReference<A> {
    int hash;
    
    *(A o) {
      super(o);
      hash = o.hashCode();
    }
    
    *(A o, ReferenceQueue<A> queue) {
      super(o, queue);
      hash = o.hashCode();
    }
    
    public int hashCode() { ret hash; }
    public bool equals(O o) {
      ret o instanceof MyWeakReference
        && eq(get(), ((MyWeakReference) o).get());
    }
  }
  
  // Make sure we get reaped once a minute (or whatever the policy is)
  *() {
    _registerWeakCollection(this);
  }
  
  void reap() {
    MyWeakReference ref;
    while ((ref = (MyWeakReference) queue.poll()) != null)
      set.removeIfSame(ref);
  }
  
  public int size() { reap(); ret set.size(); }
  
  public Iterator<A> iterator() {
    final Iterator<MyWeakReference<A>> it = set.iterator();
    ret iteratorFromFunction(() -> {
      while (it.hasNext()) {
        MyWeakReference<A> ref = it.next();
        A a = ref.get();
        if (a != null) ret a;
        // todo: clean up entry
      }
      null;
    });
  }
  
  public bool contains(O o) {
    reap();
    ret set.contains(new MyWeakReference(o));
  }
  
  public bool add(A a) {
    reap();
    ret set.add(new MyWeakReference(a, queue));
  }
  
  public bool remove(O o) {
    reap();
    ret set.remove(new MyWeakReference(o));
  }
  
  A find(O o) {
    reap();
    MyWeakReference<A> ref = set.find(new MyWeakReference(o));
    ret ref == null ? null : ref.get();
  }
}

Began life as a copy of #1011596