def delete_lower(script, layer_num=None):
    """ Delete all layers below the specified one.

    Useful for MeshLab ver 2016.12, whcih will only output layer 0.
    """
    if layer_num is None:
        layer_num = script.current_layer()
    if layer_num != 0:
        change(script, 0)
    for i in range(layer_num):
        delete(script, 0)
    return None