@NotNull
    @Override
    public <T> It<T> createIterator(@NotNull Iterable<? extends T> roots,
                                    @NotNull Function<? super T, ? extends Iterable<? extends T>> tree) {
      return new OnlyLoadedBfsIt<>(roots, tree);
    }
@NotNull
    @Override
    public <T> It<T> createIterator(@NotNull Iterable<? extends T> roots,
                                    @NotNull Function<? super T, ? extends Iterable<? extends T>> tree)