human
Solve the following python programming problem given the following code:
def test_linear_2d() : 
    ' should work like a linear KF if problem is linear ' 
    def fx(x, dt) : 
        F = np.array ([[1, dt, 0, 0], [0, 1, 0, 0], [0, 0, 1, dt], [0, 0, 0, 1]], dtype = float) 
        return np.dot (F, x) 
def hx(x) : 
        return np.array ([x [0], x [2]]) 
dt = 0.1 
    points = MerweScaledSigmaPoints (4, 0.1, 2.0, (- 1)) 
    kf = UKF (dim_x = 4, dim_z = 2, dt = dt, fx = fx, hx = hx, points = points) 
    kf.x = np.array ([(- 1.0), 1.0, (- 1.0), 1]) 
    kf.P *= 0.0001 
    zs = [] 
    for i in range (20) : 
        z = np.array ([(i + (randn () * 0.1)), (i + (randn () * 0.1))]) 
        zs.append (z) 
(Ms, Ps) = kf.batch_filter (zs) 
    (smooth_x, _, _) = kf.rts_smoother (Ms, kf, dt = dt) 
    if DO_PLOT : 
        zs = np.asarray (zs) 
        plt.plot (Ms [:, 0]) 
        plt.plot (smooth_x [:, 0], smooth_x [:, 2]) 
        print (smooth_x) 

    
    

    Fix the buggy line: (smooth_x, _, _) = kf.rts_smoother (Ms, kf, dt = dt)
assistant
#Here is Python code to fix the buggy line: (smooth_x, _, _) = kf.

(smooth_x, _, _) = kf.rts_smoother (Ms, Ps, dt = dt)