def swap_off_diagonal(cube, f, l1, l2):
    """
    A big-cube move that swaps three cubies (I think) but looks like two.
    """
    cube.move(f, l1, 1)
    cube.move(f, l2, 1)
    cube.move('U', 0, -1)
    cube.move(f, l2, -1)
    cube.move('U', 0, 1)
    cube.move(f, l1, -1)
    cube.move('U', 0, -1)
    cube.move(f, l2, 1)
    cube.move('U', 0, 1)
    cube.move(f, l2, -1)
    return None
