def to_set(labels_list):
    """given a list of labels from annotations, return the set of (unique) labels

    Parameters
    ----------
    labels_list : list
         of lists, i.e. labels from annotations

    Returns
    -------
    labelset

    Examples
    --------
    >>> labels_list = [voc.annot.labels for voc in vds.voc_list]
    >>> labelset = to_set(labels_list)
    """
    all_labels = [lbl for labels in labels_list for lbl in labels]
    labelset = set(all_labels)
    return labelset