code: public Iterator<QueueUser> getUsers() {
          if (users == null) {
              return new HashSet<QueueUser>().iterator();
          }
          return Collections.unmodifiableSet(users).iterator();
      }
docs: Returns an Iterator for the users in the queue waiting to be routed to
  an agent (QueueUser instances).
  
  @return an Iterator for the users waiting in the queue.