Returns a label if all the elements of an overlay agree on a
        consistent label, otherwise returns the default label.

def get_overlay_label(cls, overlay, default_label=''):
        """
        Returns a label if all the elements of an overlay agree on a
        consistent label, otherwise returns the default label.
        """
        if all(el.label==overlay.get(0).label for el in overlay):
            return overlay.get(0).label
        else:
            return default_label