def rucklidge(XYZ, t, k=2, a=6.7):
    """
    The Rucklidge Attractor.
    x0 = (0.1,0,0)
    """
    x, y, z = XYZ
    x_dt = -k * x + y * (a - z)
    y_dt = x
    z_dt = -z + y**2
    return x_dt, y_dt, z_dt