public final @NotNull <T> JBIterable<T> traversal(final @Nullable T root, final @NotNull Function<? super T, ? extends Iterable<? extends T>> tree) {
    return traversal(JBIterable.of(root), tree);
  }
public final @NotNull <T> JBIterable<T> traversal(final @Nullable T root, final @NotNull Function<? super T, ? extends Iterable<? extends T>> tree)