private @NotNull FreshChildrenSet acquireFreshChildrenSet() {
    var ref = freshChildrenSet.get();
    var result = ref.get();
    if (result == null) {
      result = new FreshChildrenSet();
      ref.set(result);
    }
    else {
      result.acquire();
    }
    return result;
  }
private @NotNull FreshChildrenSet acquireFreshChildrenSet()