static Set<S> makeEleuSet(final S className) {
  // It's a "degenerate" set that only supports contains().
  
  ret new AbstractSet<S>() {
    public Iterator<S> iterator() { ret null; }
    public int size() { ret -1; }
    public boolean contains(O o) {
      ret matchStart("yes", askSelf(format("is * " + a(className), ((S) o))));
    }
  };
}