// TODO: does not detect set type (hash/tree) when it's synchronized
static <A> Set<A> cloneSet(Set<A> set) {
  // assume mutex is equal to collection, which will be true unless you explicitly pass a mutex to synchronizedList() which no one ever does.
  synchronized(set) {
    ret set instanceof TreeSet ? new TreeSet(set) : new HashSet(set);
  }
}