def _varargs_labels_as_list(label_list):
    """Return a list of labels for a list of labels or singleton list of list
    of labels."""
    if len(label_list) == 0:
        return []
    elif not _is_non_string_iterable(label_list[0]):
        # Assume everything is a label.  If not, it'll be caught later.
        return label_list
    elif len(label_list) == 1:
        return label_list[0]
    else:
        raise ValueError("Labels {} contain more than list.".format(label_list),
                         "Pass just one list of labels.")
Return a list of labels for a list of labels or singleton list of list
    of labels.