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