anchor.diff:        var remaining = cb.nextCall - offset;
   
         var t = _zone.parent.createTimer(remaining, () {
  -        var next = _callbacks.first;
  -        _callbacks.remove(next);
  +        var next = _popCallback();
   
           if (next.isPeriodic) {
             var pt = _zone.parent.createPeriodicTimer(cb.duration, (_) {
  -            var next = _callbacks.first;
  -            _callbacks.remove(next);
  -            _callbacks.add(next.next());
  -            next.callback(_SimpleCallbackTimer(next.sequence, this));
  +            var next = _popCallback().next();
  +            _callbacks.add(next);
  +            next.callback(_CallbackTimer(next.sequence, this));
             });
  -          _timers.add(pt);
             next._timer = pt;
  +          _callbacks.remove(next);
             _callbacks.add(next.next());
           }
   
           next.callback();
         });
   
  -      _timers.add(t);
         cb._timer = t;
       }
     }
  
positive.code: 
    void _cancel(int id) {
      // TODO: sorted map by sequence
      var cb = _callbacks.firstWhere((t) => t.sequence == id);
      cb._timer.cancel();
      _callbacks.remove(cb);
      // TODO: also remove timer associated with id
    }
  
    bool _isActive(int id) => _callbacks.any((t) => t.sequence == id);
  
    int _ticks(int id) {
      throw UnimplementedError();
  
negative.code: 
  class _CallbackQueueZone implements PausableZone {
    final _callbacks = SplayTreeSet<_ScheduledTimer>();
  
    /// The offset of the parent zone, which progresses independently of pauses.
    ///
  