public final @NotNull <T> Function<T, JBIterable<T>> traversal(final @NotNull Function<? super T, ? extends Iterable<? extends T>> tree) {
    return t -> traversal(t, tree);
  }
public final @NotNull <T> Function<T, JBIterable<T>> traversal(final @NotNull Function<? super T, ? extends Iterable<? extends T>> tree)