sclass QPool {
  replace Q with CompactQ.
  
  int max = numberOfCores();
  new SelfNotifyingSet<Q> used;
  new SelfNotifyingSet<Q> free;
  
  synchronized int total() { ret l(used)+l(free); }
  synchronized bool nonFree() { ret empty(free) && max < total(); }
  
  event customerMustWaitAlert;
  
  void fireCustomerMustWaitAlert {
    vmBus_send customerMustWaitAlert(this, currentQ());
    customerMustWaitAlert();
  }
  
  synchronized WithAutoCloseable<Q> acquireQ() {
    if (nonFree())
      fireCustomerMustWaitAlert();
    if (total() < max)
      used.add(newQ());
    sleepWhileEmpty(free);
    Q t = assertNotNull(popFirst(free));
    used.add(t);
    ret withAutoCloseable(-> giveQBack(t), t);
  }
  
  synchronized void giveQBack(Q t) {
    used.remove(t);
    free.add(t);
  }
  
  static Q newQ() {
    ret new Q("Q Pool Inhabitant " + n2(total()+1)) {
      
    };
  }
}