sclass CompactQ extends Meta is AutoCloseable {
  // Step 1 - no name... well, no reserved name field, anyway.
  // Most queues can be probably identified from context.
  // if you want æ name - use Meta
  
  S name() { ret or(2(S) metaGet("name"), toString()); }
  void name(S name) { metaSet(+name); }
  
  // Np syncLinkedList - just an AppendableChain
  // All sync is done on the main object
  AppendableChain<Runnable> q;
  
  // retired also goes to meta
  bool retired() { ret metaGet("retired") != null; }
  void retired(bool b) { metaPut("retired", trueOrNull(b)); }
  
  // optional thread ownership marker, e.g. for OS modules.
  // Having slashed absolutely everything else, we do keep that as an actual field
  IF0<AutoCloseable> enter;
  
  // We do want to know who is servicing us currently...
  Thread thread;
  
  AutoCloseable enter() { ret callF(enter); }
  
  synchronized bool running() { ret thread != null; }

  bool done() { ret retired() || !licensed() || isEmpty(); }
  
  void waitUntilDone {
    while (!done()) sleep(1);
  }
  
  synchronized void cancel aka close() {
    clear();
    if (thread == null) ret;
    threadBeingCancelled = new WeakReference(thread);
    cancelAndInterruptThread(thread);
    thread = null;
  }
  
  close {
    retired(true);
    clear();
    cancelAndInterruptThread();
  }
  
  public synchronized void clear {
    q = null;
  }

  // append to q (do later)
  synchronized void add(Runnable r) {
    if (r == null) ret;
    q = chainPlus(q, r);
    checkForAction();
  }
  
  // prepend to q (do next)
  synchronized void addInFront(Runnable r) {
    if (r == null) ret;
    q = itemPlusChain(r, q);
    checkForAction();
  }
  
  protected void checkForAction() {
    if (!done() && !running()) {
      temp callF(enter);
      thread = startThread(name, r {
        temp callF(enter);
        _bout();
      });
    }
  }

  // work as long there is work
  void _bout {
    while (licensed() && retired()) {
      Runnable r;
      synchronized {
        r = first(q);
        q = popFirst(q);
      }
      if (r != null)
        pcall { r?.run(); }
      else
        onIdle();
    }
  }
  
  bool isEmpty() { ret q == null; }
  int size() { ret l(q); }
  
  O mutex() { this; } // clients can synchronize on this
  L<Runnable> snapshot() { ret cloneList(q); }
  
  // you can override this
  void onIdle {}
}