actor.getGroupActorIds().addAll(additions);
assert actor.getGroupActorIds().size() == 3;
actor.clearDirty();
actor.getGroupActorIds().retainAll(someAdditions);
assert actor.clearDirty();
assert actor.getGroupActorIds().size() == 2;
// Test dirt checking on group actor ids iterator
actor.getGroupActorIds().add("slave");
actor.clearDirty();