Computes a title for bokeh tabs from the key in the overlay, the
    element and the containing (Nd)Overlay.

def get_tab_title(key, frame, overlay):
    """
    Computes a title for bokeh tabs from the key in the overlay, the
    element and the containing (Nd)Overlay.
    """
    if isinstance(overlay, Overlay):
        if frame is not None:
            title = []
            if frame.label:
                title.append(frame.label)
                if frame.group != frame.params('group').default:
                    title.append(frame.group)
            else:
                title.append(frame.group)
        else:
            title = key
        title = ' '.join(title)
    else:
        title = ' | '.join([d.pprint_value_string(k) for d, k in
                            zip(overlay.kdims, key)])
    return title