Computes the z-order of element in the NdOverlay
        taking into account possible batching of elements.

def get_zorder(self, overlay, key, el):
        """
        Computes the z-order of element in the NdOverlay
        taking into account possible batching of elements.
        """
        spec = util.get_overlay_spec(overlay, key, el)
        return self.ordering.index(spec)