sclass PlanInMotion<A> extends DynamicObject {
  L<A> doneSteps = synchroList();
  L<A> plannedSteps = synchroLinkedList();
  volatile bool paused, cancelled;
  
  void add(A a) { plannedSteps.add(a); }
  
  A next() { ret syncPopFirst(plannedSteps); }
}