def add_repeat(vstruct, repeats=(0, 0, 0)):
    """repeat all elements by their local centre and cell vectors
    vector : ['a','b','c']
    rep : int
    recentre: bool
        if True, move centre by 0.5 * rep * vector
    """
    vstruct['transforms'].append({'type': 'local_repeat', 'repeats': repeats})
