sclass PlanInMotion<A> extends DynamicObject {
  L<A> doneSteps = synchroList();
  L<A> plannedSteps = synchroLinkedList();
  volatile bool paused, cancelled;
  
  void add(A a) { plannedSteps.add(a); }
  
  IterableIterator<A> master() {
    ret iteratorFromFunction(func -> A {
      paused || cancelled ? null : syncPopFirst(plannedSteps)
    });
  }
}